Skip to main content

Full text of "Moritz Schlick-Positivism and Realism"

See other formats


Chapter 1 

Positivism and Realism 

Moritz Schlick 



I. Preliminary Questions 

Every philosophical movement is defined by the principles that it regards as funda- 
mental and to which it constantly recurs in its arguments. But in the course of 
historical development, the principles are apt not to remain unaltered, whether it be 
that they acquire new formulations, and come to be extended or restricted, or that 
even their meaning gradually undergoes noticeable modifications. At some point the 
question then arises as to whether we should still speak at all of the development of a 
Single movement and retain its old name, or whether a new movement has not in fact 
arisen. 

If, alongside the evolved outlook, an 'orthodox' movement still continues to exist, 
which clings to the first principles in their original form and meaning, then sooner or 
later some terminological distinction of the old from the new will automatically come 
about. But where this is not clearly so, and where, on the contrary, the most diverse 
and perhaps contradictory formulations and interpretations of the principles are ban- 
died about among the various adherents of a 'movement', then a hubbub arises, 
whose result is that supporters and opponents of the view are found talking at cross 
purposes; every one seeks out from the principles what he can specifically use for the 
defense of his own view, and everything ends in hopeless misunderstandings and 
obscurities. They only disappear when the various principles are separated from each 
other and tested individually for meaning and truth on their own account, in which 
process we do best, at first, to disregard entirely the contexts in which they have 
historically arisen, and the names that have been given to them. 

I should like to apply these considerations to the modes of thought grouped under 
the name of 'positivism'. From the moment when Auguste Comte invented the term, 
up to the present day, they have undergone a development which provides a good- 
example of what has just been said. I do this, however, not with the historical purpose 
of establishing, say, a rigorous concept of positivism in its historical manifestation, but 
rather in order to contribute to a real settlement of the controversy currently carried 
on about certain principles which rarik as positivist axioms. Such a settlement is all the 
dearer to me, in that I subscribe to some of these principles myself. My only concem 
here is to make the meaning of these principles as clear as possible; whether, after such 
clarificatioa people are still minded to impute them to 'positivism' or not, is a question 
of whoUy subordinate importance. 

If every view is to be labelled positivist, which denies the possibility of metaphysics, 
then nothing can be said against it as a mere definition, and in this sense I would have 

Originally appeared in Erkenntnis III (1932/33); translated by Peter Heath and reprinted in Moritz Schlick: 
Philosophical Papers, Volume II {1925-1936) from Vienna Circle Collection, edited by Henk L. Mulder 
(Kluwer, 1979), pp. 259-284. Reprinted by permission of the Schlick estate and the publisher. Copyright 
1979 by Kluwer Academic Publishers. 



38 Moritz Schlick 

to declare myself a strict positivist. But this, of course, is true only if we presuppose a 
particular definition of metaphysics'. What the definition of metaphysics is, that would 
have to be made basic here, does not need to interest us at present; but it scarcely 
accords with the formulations that are mostly current in the literature of philosophy; 
and closer definitions of positivism that adhere to such formulations lead straight into 
obscurities and difficulties. 

For if, say — as has mostly been done from time immemorial — we assert that 
metaphysics is the doctrine of 'true being', of 'reality in itself, or of 'transcendent 
being', this talk of true, real being obviously presupposes that a non-true, lesser or 
apparent being Stands opposed to it, as has indeed been assumed by all metaphysicians 
since the days of Plato and the Eleatics, This seeming being is said to be the realm of 
appearances', and while the true transcendent reality is held to be accessible with 
difficulty only to the efforts of the metaphysician, the special sciences are exclusively 
concemed with appearances, and the latter are also perfectly accessible to scientific 
knowledge. The contrast in the knowability of the two 'kinds of being' is then traced 
to the fact that appearances are given' and immediately known to us, whereas meta- 
physical reality has had to be inferred from them only by a circuitous route. With this 
we seem to have arrived at a fundamental concept of the positivists, for they, too, are 
always talking of the given', and state their basic principle mostly by saying that, like 
the scientist, the philosopher must abide throughout in the given, that an advance 
beyond it, such as the metaphysician attempts, is impossible or absurd. 

It is natural, therefore, to take the given of positivism to be simply identical with the 
metaphysician's appearances, and to believe that positivism is at bottom a metaphysics 
from which the transcendent has been omitted or Struck out; and such a view may 
often enough have inspired the arguments of positivists, no less than those of their 
adversaries. But with this we are already on the road to dangerous errors. 

This very term 'the given' is already an occasion for grave misunderstandings. 'To 
give', of course, normally signifies a three-termed relation: it presupposes in the first 
place someone who gives, secondly someone given to, and thirdly something given. 
For the metaphysician this is quite in order, for the giver is transcendent reality, the 
receiver is the knowing consciousness, and the latter appropriates what is given to it 
as its 'content'. But the positivist, from the outset, will obviously have nothing to do 
with such notions; the given, for him, is to be merely a term for what is simplest and 
no longer open to question. Whatever term we may choose, indeed, it will be liable to 
occasion misconceptions; if we talk of 'acquaintance' [' Erlebnis l we seem to presuppose 
the distinction between he who is acquainted and what he is acquainted with; in 
employing the term 'content of consciousness', we appear to bürden ourselves with 
a similar distinction, and also with the complex concept of 'consciousness', first ex- 
cogitated, at all events, by philosophical thought. 

But even apart from such difficulties, it is possibly still not yet clear what is actually 
meant by the given. Does it merely include such 'qualities' as 'blue', 'hot' and 'pain', or 
also, for example, relations between them, or the order they are in? Is the similarity of 
two qualities 'given in the same sense as the qualities themselves? And if the given is 
somehow elaborated or interpreted or judged, is this elaboration or judgement not also 
in tum a given in some sense? 

It is not obscurities of this type, however, which give occasion to present-day 
controversies; it is the question of 'reality' that first tosses among the parties the apple 
of discord. 

If positivism's rejection of metaphysics amounts to a denial of transcendent reality, 
it seems the most natural thing in the world to conclude that in that case it attributes 



Positivism and Realism 39 

reality only to non-transcendent being. The main principle of the positivist then seems 
to run: 'Only the given is real'. Anyone who takes pleasure in plays upon words could 
even make use of a peculiarity of the Gennan language in order to lend this proposi- 
tion the air of being a self-evident tautology, by formulating it as: 'Es gibt nur das 
Gegebene [Only the given exists]. 

What are we to say of this principle? 

Many positivists may have stated and upheld it (particularly those, perhaps, who 
have treated physical objects as 'mere logical constructions' or as mere auxiliary 
concepts'), and others have had it imputed to them by opponents — but we are obliged 
to say that anyone who asserts this principle thereby attempts to advance a claim that 
is metaphysical in the same sense, and to the same degree, as the seemingly opposite 
contention, that There is a transcendent reality'. 

The problem at issue here is obviously the so-called question as to the reality of the 
extemal world and on this there seem to be two parties: that of realism', which 
believes in the reality of the externa! world, and that of 'positivism', which does not 
believe in this. I am convinced that in fact it is quite absurd to set two views in contrast 
to one another in this fashion, since (as with all metaphysical propositions) both 
parties, at bottom, have not the least notion of what they are trying to say. But before 
explaining this I should like to show how the most natural interpretations of the 
proposition 'only the given is real' in fact lead at once to familiär metaphysical views. 

As a question about the existence of the 'extemal' world, the problem can make its 
appearance only through drawing a distinction of some kind between inner and outer, 
and this happens inasmuch and insofar as the given is regarded as a 'content' of 
consciousness, as belonging to a subject (or several) to whom it is given. The immedi- 
ate data are thereby credited with a conscious character, the character of presentations 
or ideas; and the proposition in question would then assert that all reality possesses 
this character: no being outside consciousness. But this is nothing eise but the basic 
principle of meta-physical idealism, If the philosopher thirJcs he can speak only of 
what is given to himself, we are confronted with a solipsistic metaphysics; but if he 
thinks he may assume that the given is distributed to many subjects, we then have an 
idealism of the Berkeleyan type. 

On this interpretation, positivism would thus be simply identical with the older 
idealist metaphysics. But since its founders were certainly seeking something quite 
other than a renewal of that idealism, this view must be rejected as inconsistent with 
the antimetaphysical purpose of positivism, Idealism and positivism do not go to- 
gether, The positivist Ernst Laas^ devoted a work in several volumes to demonstrating 
the irreconcilable Opposition that exists between them in all areas; and if his pupil Hans 
Vaihinger gave his Philosophy of As If the subtitle of an 'idealist positivism', that is 
just one of the contradictions that infect this work. Ernst Mach has particularly 
emphasized that his own positivism has evolved in a direction away from the Berke- 
leyan metaphysics; he and Avenarius laid much stress on not construing the given as 
a content of consciousness, and endeavored to keep this notion out of their philosophy 
altogether. 

In view of the uncertainty in the positivists' own camp, it is not surprising if the 
'realist' ignores the distinctions we have mentioned and directs his arguments against 
the thesis that 'there are only contents of consciousness', or that 'there is only an 
internal world'. But this proposition belongs to the idealist metaphysics; it has no place 
in an antimetaphysical positivism, and these counter-arguments do not teil against 
such a view. 

The 'realist' can, indeed, take the line that it is utterly inevitable that the given 



40 Moritz Schlick 

should be regarded as a content of consciousness, as subjective, or mental — or what- 
ever the term may be; and he would consider the attempts of Avenarius and Mach to 
construe the given as neutral and to do away with the inner-outer distinction, as a 
failure, and would think a theory without metaphysics to be simply impossible. But 
this line of argument is more rarely encountered. And whatever the position there, we 
are dealing in any case with a quarrel about nothing, since the problem of the reality 
of the externa! world' is a meaningless pseudo-problem. It is now time to make this 
clear. 

//. Ort the Meaning of Statements 

It is the proper business of philosophy to seek for and clarify the meaning of claims and 
questions. The chaotic state in which philosophy has found itself throughout the 
greatest part of its history is traceabie to the unlucky fact that firstly it has accepted 
certain formulations with far too much naivete, as genuine problems, without first 
carefuUy testing whether they really possessed a sound meaning; and secondly, that it 
has believed the answers to certain questions to be discoverable by particular philo- 
sophical methods that differ from those of the special sciences. By philosophical 
analysis we are unable to decide of anything whether it is real; we can only determine 
what it means to claim that it is real; and whether this is then the case or not can only 
be decided by the ordinary methods of daily life and science, namely by experience. 
So here the task is to get clear whether a meaning can be attached to the question 
about the reality of the externa] world'. 

When are we certain, in general that the meaning of a question is clear to us? 
Obviously then, and only then, when we are in a position to state quite accurately the 
circumstances under which it can be answered in the affirmative — or those under 
which it would have to receive a negative answer, By these Statements, and these 
alone, is the meaning of the question defined. 

It is the first step in every kind of philosophizing, and the basis of all reflection, to 
realize that it is absolutely impossible to give the meaning of any claim save by 
describing the state-of-affairs that must obtain if the claim is to be true. If it does not 
obtain, then the claim is false. The meaning of a proposition obviously consists in this 
alone, that it expresses a particular state-of-affairs. This state-of-affairs must actually 
be pointed out, in order to give the meaning of the proposition. One may say, indeed, 
that the proposition itself already gives this state-of-affairs; but only, of course, for one 
who understanäs it. But when do I understand a proposition? When I know the meaning 
of the words that occur in it? This can be explained by definitions. But in the definitions 
new words occur, whose meaning I also have to know in tum. The business of defining 
cannot go on indefinitely, so eventually we come to words whose meaning cannot 
again be described in a proposition; it has to be pointed out directly; the meaning of 
the word must ultimately be shown, it has to be given. This takes place through an act 
of pointing or showing, and what is shown must be given, since otherwise it cannot 
be pointed out to me. 

In order, therefore, to find the meaning of a proposition, we have to transform it 
by introduction of successive definitions, until finally only such words appear in it as 
can no longer be defined, but whose meanings can only be indicated directly. The 
criterion for the truth or falsity of the proposition then consists in this, that under 
specific conditions (stated in the definitions) certain data are, or are not, present. Once 
this is established, I have established everything that the proposition was talking 
about, and hence I know its meaning. If I am not capable, in principle, of verifying a 



Positivism and Realism 41 

proposition, that is, if I have absolutely no knowledge of how I should go about it, 
what I would have to do, in order to ascertain its truth or falsity, then I obviously have 
no idea at all of what the proposiHon is actually saying; for then 4 would be in no 
Position to interpret the proposition, in proceeding, by means of the definitions, from 
its wording to possible data, since insofar as l am in a position to do this, I can also, 
by this very fact, point out the road to verification in principle (even though, for 
practica! reasons, I may often be unable actually to tread it). To state the circumstances 
under which a proposition is true is the same as stating its meaning, and nothing eise. 

And these circumstances', as we have now seen, have ultimately to be found in the 
given. Different circumstances imply differences in the given. The meaning of every 
proposition is ultimately determined by the given alone, and by absolutely nothing 
eise. 

I do not know if this view should be described as positivistic; though I should like 
to believe that it has been in the background of all efforts that go under this name in 
the history of philosophy, whether, indeed, it has been clearly formulated or not. It 
may well be assumed to constitute the true core and driving force of many quite 
erroneous formulations that we find among the positivists. 

Anyone who has once attained the insight, that the meaning of any statement can 
be determined only by the given, no longer even grasps the possibility of another 
opinion, for he sees that he has merely discemed the conditions under which opinions 
can be formulated at all. It would thus be quite erroneous as well to perceive in the 
foregoing any sort of 'kheory of meaning' (in Anglo-Saxon countries the view outlined, 
that the meaning of a statement is whoUy and solely determined by its verification in 
the given, is commonly called the experimental theory of meaning'); that which 
precedes all formation of theories cannot itself be a theory. 

The content of our thesis is in fact entirely trivial (and that is precisely why it can 
give so much insight); it teils us that a statement only has a specifiable meaning if it 
makes some testable difference whether it is true or false. A proposition for which the 
World looks exactly the same when it is true as it does when it is false, in fact says 
nothing whatever about the world; it is empty, it conveys nothing, I can specify no 
meaning for it. But a testable difference is present only if there is a difference in the 
given, for to be testable certainly means nothing eise but 'demonstrable in the given'. 

It is self-evident that the term 'testability' is intended only in principle, for the 
meaning of a proposition does not, of course, depend on whether the circumstances 
under which we actually find ourselves at a given moment allow of. or prevent actual 
verification. The statement that 'there are 10,000 ft mountains on the far side of the 
moon' is beyond doubt absolutely meaningful, although we lack the technical means 
for verifying it. And it would remain just as meaningful even if we knew for certain, 
on scientific grounds of some kind, that no man would ever reach the far side of the 
moon. Verification always remains thinkahle, we are always able to say what sort of 
data we should have to encounter, in order to effect the decision; it is logically possible, 
whatever the Situation may be as regards the actual possibility of doing it. And that is 
all that is at issue here. 

But if someone advanced the claim, that within every electron there is a nucleus 
which is always present, but produces absolutely no effects outside, so that its exis- 
tence in nature is discemible in no way whatever — then this would be a meaningless 
claim. For we should at once have to ask the fabricator of this hypothesis: What, then, 
do you actually mean by the presence of this 'nucleus'?, and he could only reply: I mean 
that something exists there in the electron. We would then go on to ask: What is that 
supposed to mean? How would it be if this something did not exist? And he would 



42 Moritz Schlick 

have to reply: In that case, everything eise would be exactly as before. For according 
to his claim, no effects of any kind proceed from this something, and everything 
observable would remain absolutely unaltered, the realm of the given would not be 
touched. We would judge that he had not succeeded in conveying to us the meaning 
of his hypothesis, and that it is therefore vacuous. In this case the impossibility of 
verification is actually not a factual, but a logical impossibility, since the claim that this 
nucleus is totally without effects rules out, in principle, the possibility of deciding by 
differences in the given. 

Nor can it be supposed that the distinction between essential impossibility of 
verification and a merely factual and empirical impossibility is not sharp, and therefore 
often hard to draw; for the essential' impossibility is simply a logical one, which differs 
from the empirical not by degrees, but absolutely. What is merely empirically impos- 
sible still remains thinkable; but what is logically impossible is contradictory, and 
cannot, therefore, be thought at all. We also find, in fact, that with sure instinct, this 
distinction is always very clearly sensed in the practice of scientific thinking. The 
physicists would be the first to reject the claim in our example, conceming the etemally 
hidden nucleus of the electron, with the criticism that this is no hypothesis whatever, 
but an empty play with words. And on the question of the meaning of their State- 
ments, successful students of reality have at all times adopted the standpoint here 
outlined, in that they acted upon it, even though mostly unawares. 

Thus our Position does not represent anything stränge and peculiar for science, but 
in a certain sense has always been a self-evident thing. It could not possibly have been 
otherwise, because only from this standpoint can the truth of a statement be tested at 
all; since all scientific activity consists in testing the truth of Statements, it constantly 
acknowledges the correctness of our viewpoint by what it does. 

If express confirmation be still needed, it is to be found with the utmost clarity at 
critical points in the development of science, where research is compelled to bring its 
self-evident presuppositions to consciousness. This Situation occurs where difficulties 
of principle give rise to the suspicion that something may not be in order about these 
presuppositions. The most celebrated example of this kind, which will forever remain 
notable, is Einstein's analysis of the concept of time, which consists in nothing eise 
whatever but a statement of the meaning of our assertions about the simultaneity of 
spatially separated events. Einstein told the physicists (and philosophers): you must 
first say what you mean by simultaneity, and this you can only do by showing how 
the statement 'two events are simultaneous' is verified. But in so doing you have then 
also established the meaning fully and without remainder. What is true of the simultane- 
ity concept holds good of every other; every statement has a meaning only insofar as 
it can be verified; it only signifies what is verified and absolutely nothing beyond this. 
Were someone to maintain that it contains more, he would have to be able to say what 
this more is, and for this he must again say what in the world would be different if he 
was wrong; but he can say nothing of the kind, for by previous assumption all 
observable differences have already been utilized in the verification. 

In the simultaneity example the analysis of meaning, as is right and proper for the 
physicist, is carried only so far that the decision about the truth or falsity of a temporal 
statement resides in the occurrence or non-occurrence of a certain physical event (for 
example, the coincidence of a pointer with a scale-mark); but it is clear that one may 
go on to ask: What, then, does it mean to claim that the pointer indicates a particular 
mark on the scale? And the answer to this can be nothing eise whatever but a reference 
to the occurrence of certain data, or, as we are wont to say, of certain 'sensations'. This 
is also generally admitted, and especially by physicists. "For in the end, positivism will 



Positivism and Realism 43 

always be right in this", says Planck/ "that there is no other source of knowledge but 
sensations", and this statement obviously means that the truth or falsity of a physical 
assertion is quite solely dependent on the occurrence of certain sensations (which are 
a special class of the given). 

But now there will always be many inclined to say that this grants only that the 
truth of a physical statement can be tested in absoiutely no other way save by the 
occurrence of certain sensations, but that this, however, is a different thing from 
claiming that the very meaning of the statement is thereby exhaustively presented. The 
latter would have to be denied, for a proposition can contain more than allows of 
verification; that the pointer Stands at a certain mark on the scale means more than the 
presence of certain sensations (namely, the presence of a certain state-of-affairs in the 
extemal world'). 

Of this denial of the identity of meaning and verification the foUowing needs to be 
Said: 

1. Such a denial is to be found among physicists only where they leave the proper 
territory of physical statements and begin to philosophize. (In physics, obviously, we 
find only statements about the nature or behaviour of things and processes; an express 
assertion of their 'reality' is needless, since it is always presupposed.) In his own 
territory the physicist fuUy acknowledges the correctness of our point of view. We 
have already mentioned this earlier, and have since elucidated it by the example of the 
concept of simultaneity. There are, indeed, many philosophers who say: Only relative 
simultaneity can admittedly be established, but from this it does not foUow that there 
is no such thing as absolute simultaneity, and we continue, as before, to believe in it! 
There is no way of demonstrating the falsity of this claim; but the great majori ty of 
physicists are rightly of the opinion that it is meaningless. It must be emphatically 
stressed, however, that in both cases we are concemed with exactly the same Situation. 
It makes absoiutely no difference, in principle, whether I ask: Does the statement 'two 
events are simultaneous' mean more than can be verified? Or whether I ask: Does the 
statement 'the pointer indicates the fifth scale-mark' signify more than can be verified? 
The physicist who treats the two cases differently is guilty of an inconsistency. He will 
justify himself by arguing that in the second case, where the Veality of the extemal 
World' is concemed, there is philosophically far more at stake. This argument is too 
vague for us to be able to assign it any weight but we shall shortly examine whether 
anything lies behind it. 

2. It is perfectly tme that every statement about a physical object or event says 
more than is verified, say, by the once-and-for-all occurrence of an experience. It is 
presupposed, rather, that this experience took place under quite specific conditions, 
whose fulfilment can, of course, be tested in tum only by something given; and it is 
further presupposed that still other and further verifications (after-tests, confirmations) 
are always possible, which themselves of course reduce to manifestations of some kind 
in the given. In this way we can and must make allowance for sense-deceptions and 
errors, and it is easy to see how we are to classify the cases in which we would say 
that the observer had merely dreamt that the pointer indicated a certain mark, or that 
he had not observed carefuUy, and so on. Blondlot's claims about the N-rays that he 
thought he had discovered were intended, after all, to say more than that he had had 
certain Visual sensations under certain circumstances, and hence they could also be 
refuted.*^ Strictly speaking, the meaning of a proposition about physical objects is 
exhausted only by the provision of indefinitely many possible verifications, and the 
consequence of this is, that in the last resort such a proposition can never be proved 
absoiutely tme. It is generally acknowledged, indeed, that even the most assured 



44 Moritz Schlick 

propositions of science have always to be regarded merely as hypotheses, which 
remain open to further definition and improvement. This has certain consequences for 
the logical nature of such propositions, but they do not concem us here. 

Once again: the meaning of a physical statement is never defined by a Single 
isolated verification; it must be conceived, rather, as of the form: If circumstances x are 
given, data y occur, where indefinitely many circumstances can be substituted for x, 
and the proposition remains correct on every occasion (this also holds, even if the 
statement refers to a once-and-for-all occurrence — a historical event — for such an 
event always has innumerable consequences whose occurrence can be verified). Thus 
the meaning of every physical statement ultimately lies always in an endless chain of 
data; the individual datum as such is of no interest in this connection. So if a positivist 
should ever have said that the individual objects of science are simply the given 
experiences themselves, he would certainly have been quite wrong; what every scien- 
tist seeks, and seeks alone, are rather the rules which govem the connection of expe- 
riences, and by which they can be predicted. Nobody denies that the sole verification 
of natural laws consists in the fact that they provide correct predictions of this type. 
The oft-heard objection, that the immediately givea which at most can be the object 
of psychology, is now falsely to be made into an object of physics, is thereby robbed 
of its force. 

3. The most important thing to say, however, is this: If anyone thinks that the 
meaning of a proposition is not in fact exhausted by what can be verified in the given, 
but extends far beyond that, then he must at least admit that this surplus of meaning 
is utterly indescribable, unstatable in any way, and inexpressible by any language. For 
let him just try to state it! So far as he succeeds in communicaHng something of the 
meaning, he will find that the communication consists in the very fact that he has 
pointed out some circumstances that can serve for verification in the given, and he 
thereby finds our view confirmed. Or eise he may believe, indeed, that he has stated a 
meaning, but closer examination shows that his words only signify that there is still 
'something' there, though nothing whatever is said about its nature. In that case he has 
really communicated nothing; his claim is meaningless, for one cannot maintain the 
existence of something without saying of what one is claiming the existence. This can 
be brought out by reference to our example of the essentially indemonstrable nucleus 
of the electron'; but for the sake of clarity we shall analyze yet another example of a 
very fundamental kind. 

I am looking at two pieces of green paper, and establish that they have the same 
color. The proposition asserting the likeness of colour is verified, inier aha, by the fact 
that I twice experience the same color at the same time. The statement 'two patches 
of the same color are now present' can no longer be reduced to others; it is verified by 
the fact that it describes the given. It has a good meaning: by virtue of the significance 
of the words occurring in the statement, this meaning is simply the existence of this 
similarity of color; by virtue of Unguis tic usage, the sentence expresses precisely this 
experience. I now show one of the two pieces of paper to a second observer, and pose 
the question: Does he see the green just as I do? Is his color-experience the same as 
mine? This case is essentially different from the one just examined. While there the 
statement was verifiable through the occurrence of an experience of similarity, a brief 
consideration shows that here such a verification is absolutely impossible. Of course (if 
he is not color-blind), the second observer also calls the paper green; and if I now 
describe this green to him more closely, by saying that it is more yellowish than this 
wallpaper, more bluish than this billiard-cloth, darker than this plant, and so on, he will 
also find it so each time, that is, he will agree with my statements. But even though all 



Positivism and Realism 45 

his judgments about colors were to agree entirely with mine, I can obviously never 
conclude from this that he experiences 'the same quality'. It might be that on looking 
at the green paper he has an experience that I should call 'red'; that conversely, in the 
cases where I see red, he experiences green, but of course calls it 'red', and so forth. It 
might even be, indeed, that my color sensations are matched in him by experiences of 
sound or data of some other kind; yet it would be impossible in principle ever to dis- 
cover these differences between his experience and mine. We would agree completely, 
and could never differ about our surroundings, so long only (and this is absolutely the 
only precondition that has to be made) as the inner order of his experiences agrees with 
that of mine. Their 'quality' does not come into it at all; all that is required is that they 
can be brought into a sysiem in the same fashion. 

All this is doubtless uncontested, and philosophers have pointed out this Situation 
often enough. They have mostly added, however, that such subjective differences are 
indeed theoretically possible, and that this possibility is in principle very interesting, 
but that nevertheless it is 'in the highest degree probable' that the observer and I 
actually experience the same green. We, however, must say: The claim that different 
individuals experierice the same Sensation has this verifiable meaning alone, that all 
their statements (and of course all their other behavior as well) display certain agree- 
ments; hence the claim means nothing eise whatever but this. It is merely another 
mode of expression if we say that it is a question of the likeness of two Systems of 
Order. The proposition that two experiences of different subjects not only occupy the 
same place in the order of a system, but heyond thai are also qualitatively like each 
other, has no meaning for us. It is not false, be it noted, but meaningless: we have no 
idea at all what it is supposed to signify. 

Experience shows that for the majority of people it is very difficult to agree with 
this. One has to grasp that we are really concemed here with a logical impossibility of 
verification. To speak of the likeness of two data in ihe same consciousness has an 
acceptable meaning; it can be verified through an immediate experience. But if we wish 
to talk of the likeness of two data in different consciousnesses, that is a new concept; 
it has to be defined anew, for propositions in which it occurs are no longer verifiable 
in the old fashion. The new definition is, in fact, the likeness of all reactions of the two 
individuals; no other can be found. The majority believe, indeed, that no definition is 
required here; we know straight off what 'like' means, and the meaning is in both cases 
the same. But in order to recognize this as an error, we have only to recall the concept 
of simultaneity, where the Situation is precisely analogous. To the concept of 'simulta- 
neity at the same place' there corresponds here the concept of 'likeness of experiences 
in the same individual'; and to 'simultaneity at different places' there corresponds here 
the 'likeness of experiences in different individuals'. The second is in each case some- 
thing new in comparison with the first, and must be specially defined. A directly 
experienceable quality can no more be pointed out for the likeness of two greens in 
different consciousnesses than for simultaneity at different places; both must be defined 
by way of a system of relations. 

Many philosophers have tried to overcome the difficulty that seemed to confront 
them here by all sorts of speculations and thought-experiments, in that they have 
spoken, say, of a universal consciousness (God) embracing all individuals, or have 
imagined that perhaps by an artificial linkage of the nerve-systems of two people the 
sensations of the one might be made accessible to the other and could be compared — 
but all this is useless, of course, since even by such fantastical methods it is in the end 
only Contents of one and the same consciousness that are directly compared; but the 



44 Moritz Schlick 

propositions of science have always to be regarded merely as hypotheses, which 
remain open to further definition and improvement. This has certain consequences for 
the logical nature of such propositions, but they do not concem us here. 

Once again: the meaning of a physical statement is never defined by a Single 
isolated verification; it must be conceived rather, as of the form: If circumstances x are 
given, data y occur, where indefinitely many circumstances can be substituted for x, 
and the proposition remains correct on every occasion (this also holds, even if the 
statement refers to a once-and-for-all occurrence — a historical event — for such an 
event always has innumerable consequences whose occurrence can be verified). Thus 
the meaning of every physical statement ultimately lies always in an endless chain of 
data; the individual datum as such is of no interest in this connection. So if a positivist 
should ever have said that the individual objects of science are simply the given 
experiences themselves, he would certainly have been quite wrong; what every scien- 
tist seeks, and seeks alone, are rather the rules which govem the connection of expe- 
riences, and by which they can be predicted. Nobody denies that the sole verification 
of natural laws consists in the fact that they provide correct predictions of this type. 
The oft-heard objection, that the immediately given, which at most can be the object 
of psychology, is now falsely to be made into an object of physics, is thereby robbed 
of its force. 

3. The most important thing to say, however, is this: If anyone thinks that the 
meaning of a proposition is not in fact exhausted by what can be verified in the given, 
but extends far beyond that, then he must at least admit that this surplus of meaning 
is utterly indescribable, unstatable in any way, and inexpressible by any language. For 
let him just try to state it! So far as he succeeds in communicating something of the 
meaning, he will find that the communication consists in the very fact that he has 
pointed out some circumstances that can serve for verification in the given, and he 
thereby finds our view confirmed. Or eise he may believe, indeed, that he has stated a 
meaning, but closer examination shows that his words only signify that there is still 
'something' there, though nothing whatever is said about its nature. In that case he has 
really communicated nothing; his claim is meaningless, for one cannot maintain the 
existence of something without saying of what one is claiming the existence. This can 
be brought out by reference to our example of the essentially indemonstrable nucleus 
of the electron'; but for the sake of clarity we shall analyze yet another example of a 
very fundamental kind. 

I am looking at two pieces of green paper, and establish that they have the same 
color. The proposition asserting the likeness of colour is verified, inter aha, by the fact 
that I twice experience the same color at the same time. The statement 'two patches 
of the same color are now present' can no longer be reduced to others; it is verified by 
the fact that it describes the given. It has a good meaning: by virtue of the significance 
of the words occurring in the statement, this meaning is simply the existence of this 
similarity of color; by virtue of linguistic usage, the sentence expresses precisely this 
experience. I now show one of the two pieces of paper to a second observer, and pose 
the question: Does he see the green just as I do? Is his color-experience the same as 
mine? This case is essentially different from the one just examined. While there the 
statement was verifiable through the occurrence of an experience of similarity, a brief 
consideration shows that here such a verification is absolutely impossible. Of course (if 
he is not color-blind), the second observer also calls the paper green-, and if I now 
describe this green to him more closely, by saying that it is more yellowish than this 
wallpaper, more bluish than this billiard-cloth, darker than this plant and so on, he will 
also find it so each time, that is, he will agree with my statements. But even though all 



Positivism and Realism 45 

his judgments about colors were to agree entirely with mine, I can obviously never 
conclude from this that he experiences 'the same quality'. It might be that on looking 
at the green paper he has an experience that I should call red'; that conversely, in the 
cases where I see red, he experiences green, but of course calls it 'red', and so forth. It 
might even be, indeed, that my color sensations are matched in him by experiences of 
sound or data of some other kind; yet it would be impossible in principle ever to dis- 
cover these differences between his experience and mine. We would agree completely, 
and could never differ about our surroundings, so long only (and this is absolutely the 
only precondition that has to be made) as the inner order of his experiences agrees with 
that of mine. Their 'quality' does not come into it at all; all that is required is that they 
can be brought into a sysiem in the same fashion. 

All this is doubtless uncontested, and philosophers have pointed out this Situation 
often enough. They have mostly added, however, that such subjective differences are 
indeed theoretically possible, and that this possibility is in principle very interesting, 
but that nevertheless it is 'in the highest degree probable' that the observer and I 
actually experience the same green. We, however, must say: The claim that different 
individuals experierice the same Sensation has this verifiable meaning alone, that all 
their statements (and of course all their other behavior as well) display certain agree- 
ments; hence the claim means nothing eise whatever but this. It is merely another 
mode of expression if we say that it is a question of the likeness of two Systems of 
Order. The proposition that two experiences of different subjects not only occupy the 
same place in the order of a system, but heyond ihat are also qualitatively like each 
other, has no meaning for us. It is not false, be it noted, but meaningless: we have no 
idea at all what it is supposed to signify. 

Experience shows that for the majori ty of people it is very difficult to agree with 
this. One has to grasp that we are really concemed here with a logical impossibility of 
verification. To speak of the likeness of two data in ihe same consciousness has an 
acceptable meaning; it can be verified through an immediate experience. But if we wish 
to talk of the likeness of two data in different consciousnesses, that is a new concept; 
it has to be defined anew, for propositions in which it occurs are no longer verifiable 
in the old fashion. The new definition is, in fact, the likeness of all reactions of the two 
individuals; no other can be found. The majori ty believe, indeed, that no definition is 
required here; we know straight off what 'like' means, and the meaning is in both cases 
the same. But in order to recognize this as an error, we have only to recall the concept 
of simultaneity, where the Situation is precisely analogous. To the concept of 'simulta- 
neity at the same place' there corresponds here the concept of 'likeness of experiences 
in the same individual'; and to 'simultaneity at different places' there corresponds here 
the 'likeness of experiences in different individuals'. The second is in each case some- 
thing new in comparison with the first, and must be specially defined. A directly 
experienceable quality can no more be pointed out for the likeness of two greens in 
different consciousnesses than for simultaneity at different places; both must be defined 
by way of a System of relations. 

Many philosophers have tried to overcome the difficulty that seemed to confront 
them here by all sorts of speculations and thought-experiments, in that they have 
spoken, say, of a universal consciousness (God) embracing all individuals, or have 
imagined that perhaps by an artificial linkage of the nerve-systems of two people the 
sensations of the one might be made accessible to the other and could be compared — 
but all this is useless, of course, since even by such fantastical methods it is in the end 
only Contents of one and the same consciousness that are directly compared; but the 



46 Moritz Schlick 

question is precisely whether a comparison is possible between qualities insofar as they 
belong to different consciousnesses, and not the same one. 

It must be admitted, therefore, that a proposition about the likeness of the expe- 
riences of two different persons has no other stateable meaning save that of a certain 
agreement in their reactions. Now it is open to anyone to believe that such a proposi- 
tion also possesses another, more direct meaning; but it is certain that this meaning is 
not verifiable, and that there can be no way at all of stating or pointing out what this 
meaning is supposed to be. From this it foUows, however, that there is absolutely no 
way at all in which such a meaning could be made a topic of discussion; there could be 
absolutely no talk about it, and it can in no way enter into any language whereby 
we communicate with each other. 

And what has, we hope, become clear from this example, is of quite general applica- 
tion. All we can understand in a proposition is what it conveys; but a meaning can be 
communicated only if it is verifiable. Since propositions are nothing eise but a vehicie 
of communicatioa we can assign to their meaning only what can be communicated. 
For this reason I should insist that meaning' can never signify anything but 'stateable 
meaning'. 

But even if someone insisted that there was a nonverifiable meaning, this would 
actually be of no consequence whatever; for in everything he says and asks, and in 
everything that we ask him and reply to him, such a meaning can never in any way 
come to light. In other words, if such a thing were to exist, all our utterances and 
arguments and modes of behavior would still remain totally untouched by it, whether 
it was a question of daily life, of ethical or aesthetic attitude, of science of any kind, or 
of philosophy. Everything would be exactly as though there were no unverifiable 
meaning, for insofar as anything was different, it would in fact be verifiable through 
this very difference. 

That is a serious Situation, and we must absolutely demand that it be taken seriously. 
One must guard above all things against confusing the present logical impossibility 
with an empirical incapacity, just as though some technical difficulties and human 
imperfection were to blame for the fact that only the verifiable can be expressed, and 
as though there were still some little backdoor through which an unstateable meaning 
could slip into the daylight and make itself noticeable in our speech and behavior! No! 
The incommunicability is an absolute one; anyone who believes in a nonverifiable 
meaning (or more accurately, we shall have to say, imagines he believes in this) must 
still confess that only one attitude remains in regard to it: absolute silence, It would be 
of no use either to him or us, however often he asserted: 'but there is a non-verifiable 
meaning', for this statement is itself devoid of meaning, and says nothing. 

///. What Does 'Reality' Mean? What Does 'Extemal World' Mean? 

We are now prepared to make application of the foregoing to the so-called problem of 
the reality of the extemal world, 

Let US ask: What meaning has it, if the 'realist' says 'there is an extemal world'? or 
even: What meaning attaches to the claim (which the realist attributes to the positivist) 
'there is no extemal world'? 

To answer the question, it is necessary, of course, to clarify the significance of the 
words 'there is' and 'extemal world'. Let us begin with the first. 'There is x amounts to 
saying 'x is real' or 'x is actual'. So what does it mean if we attribute actuality (or reality) 
to an object? It is an ancient and very important insight of logic or philosophy, that the 
proposition 'x is actual' is totally different in kind from a proposition that attributes any 



Positivism and Realism 47 

sort of properiy to x (such as 'x is hard'). In other words, actuality, reality or existence 
is not a property. The Statement 'the dollar in my pocket is round' has a totally 
different logical form from the statement 'the dollar in my pocket is actual'. In modern 
logic this distinction is expressed by an altogether different symbolism, but it had 
already been very sharply emphasized by Kant, who, as we know, in his critique of the 
so-alled ontological proof of God's existence had correctly found the error of this 
proof in the fact that existence was treated like a property there. 

In daily life we very often have to speak of actuality or existence, and for that very 
reason it cannot be hard to discover the meaning of this talk. In a legal battle it often 
has to be established whether some document really exists, or whether this has merely 
been falsely claimed, say, by one of the parties; nor is it whoUy unimportant to me, 
whether the dollar in my pocket is merely imaginary or actually real. Now everybody 
knows in what way such a reality-claim is verified, nor can there be the least doubt 
about it; the reality of the dollar is proved by this, and this alone, that by suitable 
manipulations I fumish myself certain tactual or Visual sensations, on whose occurrence 
I am accustomed to say: this is a dollar. The same holds of the document, only there 
we should be content, on occasion, with certain statements by others claiming to have 
Seen the document, that is, to have had perceptions of a quite specific kind. And the 
'statements of others' again consist in certain acoustic, or — if they were written 
utterances — visual perceptions, There is need of no special controversy about the fact 
that the occurrence of certain sense-perceptions among the data always constitutes the 
sole criterion for propositions about the reality of a physical' object or event, in daily 
life no less than in the most refined assertions of science. That there are okapis in Africa 
can be established only by observing such animals. But it is not necessary that the 
object or event 'itself should have to be perceived, We can imagine, for example, that 
the existence of a trans-Neptunian planet might be inferred by Observation of perturba- 
tions with just as much certainty as by direct perception of a speck of light in the 
telescope. The reality of the atom provides another example, as does the back side of 
the moon. 

It is of great importance to state that the occurrence of some one particular expe- 
rience in verifying a reality-statement is often not recognized as such a verification, but 
that it is throughout a question of regularities, of law-like connections; in this way true 
verifications are distinguished from illusions and hallucinations. If we say of some 
event or object — which must be marked out by a description — that it is real, this 
means, then, that there is a quite specific connection between perceptions or other 
experiences, that under given circumstances certain data are presented. By this alone is 
it verified, and hence this is also its only stateable meaning. 

This, too, was already formulated, in principle, by Kant, whom nobody will accuse 
of positivism'. Reality, for him, is a category, and if we apply it anywhere, and claim 
of an object that it is real, then all this asserts, in Kant's opinion, is that it belongs to a 
law-govemed connection of perceptions, 

It will be Seen that for us (as for Kant; and the same must apply to any philosopher 
who is aware of his task) it is merely a matter of saying what is meant when we ascribe 
real existence to a thing in life or in science; it is in no sense a matter of correcting the 
Claims of ordinary life or of research. I must confess that I should charge with foUy and 
reject a limine every philosophical System that involved the claim that clouds and 
stars, mountains and the sea, were not actually real, that the physical world' did not 
exist, and that the chair against the wall ceases to be every time I tum my back on it, 
Nor do I seriously impute such a claim to any thinker. It would, for example, be 
undoubtedly a quite mistaken account of Berkeley's philosophy if his System were to 



48 Moritz Schlick 

be understood in this fashion. He, too, in no way denied the reality of the physical 
World, but merely sought to explain what we mean when we attribute reality to it. 
Anyone who says here that unperceived things are ideas in the mind of God is not in 
fact denying their existence, but is seeking, rather, to understand it. Even John Stuart 
Mill was not wanting to deny the reality of physical objects, but rather to explain it, 
when he declared them to be permanent possibilities of Sensation', although I do 
consider his mode of expression to have been very unsuitably chosen. 

So if positivism' is understood to mean a view that denies reality to bodies, I should 
simply have to declare it absurd; but I do not believe that such an interpretation of 
positivist opinions, at least as regards their competent exponents, would be historically 
just. Yet, however that may be, we are concemed only with the issue itself. And on 
this we have established as foUows: our principle, that the question about the meaning 
of a proposition is identical with the question about its verification, leads us to 
recognize that the claim that a thing is real is a statement about lawful connections of 
experiences; it does not, however, imply this claim to be faise. (There is therefore no 
denial of reality to physical objects in favor of sensations.) 

But opponents of the view presented profess themselves by no means satisfied with 
this assertion. So far as I can see, they would answer as follows: 'You do, indeed, 
acknowledge completely the reality of the physical world, but — as we see it — only in 
words. You simply call real what we should describe as mere conceptual constructions. 
When we use the word "reality", we mean by it something quite different from you. 
Your definition of the real reduces it to experiences; but we mean something quite 
independent of all experiences. We mean something that possesses the same in- 
dependence that you obviously concede only to the data, in that you reduce every- 
thing eise to them, as the not-further-reducible'. 

Although it would be a sufficient rebuttal to request our opponents to reflect once 
more upon how reality-statements are verified, and how verification is connected with 
meaning, I do in fact recognize the need to take account of the psychological attitude 
from which this argument Springs, and therefore beg attention to the foUowing 
considerations, whereby a modification of this attitude may yet, perhaps, be effected. 

Let US first enquire whether, on our view, a content of consciousness' is credited 
with a reality that is denied to a physical object. We ask, therefore: does the claim that 
a feeling or Sensation is real have a meaning different from the claim that a physical 
object is real? For us, this can mean only: are different types of verification involved in 
the two cases? The answer is: no! 

To clarify this, we need to enter a little into the logical form of reality-statements. 
The general logical recognition that an existence-statement can be made about a datum 
only if it is marked out by a description, but not if it is given by an immediate 
indication, is also valid, of course, for the 'data of consciousness'. In the language of 
symbolic logic, this is expressed by the fact that an existence-claim must contain an 
Operator . In Russell's notation, for example, a reality-statement has the form (3x)fx, or 
in words, 'there is an x that has the property /'. The form of words 'there is a, where 
'a is supposed to be the individual name of a directly indicated object, therefore means 
no more than 'this here'; this form of words is meaningless, and in Russell's symbolism 
it cannot even be written down. We have to grasp the idea that Descartes's proposi- 
tion 'I am' — or, to put it better, 'contents of consciousness exist' — is absolutely 
meaningless; it expresss nothing, and contains no knowledge. This is due to the fact 
that 'contents of consciousness' occurs in this connection as a mere name for the given; 
no characteristic is asserted, whose presence could be tested. A proposition has mean- 
ing, and is verifiable, only if I can state under what circumstances it would be true, and 



Positivism and Realism 49 

under what circumstances it would be false. But how am I to describe the circumstances 
under which the proposition 'My contents of consciousness exist' would be false? 
Every attempt would lead to ridiculous absurdities, to such propositions, say, as 'It is 
the case that nothing is the case', or the like. Hence I am self-evidently unable to 
describe the circumstances that make the proposition true (just try it!). Nor is there any 
doubt whatever that Descartes, with his proposition, had really obtained no knowl- 
edge, and was actually no wiser than before. 

No, the question about the reality of an experience has meaning only where this 
reality can also be meaningfuUy douhted. I can ask, for example: Is it really true that I 
feit joy on hearing that news? This can be verified or falsified exactly as when we ask, 
say: Is it true that Sirius has a companion (that this companion is real)? That I feit joy 
on a particular occasion can be verified, for example, by examination of other people's 
Statements about my behaviour at the time, by my finding of a letter that I then wrote, 
or simply by the retum to me of an exact memory of the emotion I experienced. Here, 
therefore, there is not the slightest difference of principle: to be real always means to 
stand in a definite connection with the given. Nor is it otherwise, say, with an 
experience that is present at this very moment. I can quite meaningfuUy ask, for 
example (in the course, say, of a physiological experiment): Do I now actually feel a 
pain or not? (Notice that pain , here, does not function as an individual name for a 'this 
here', but represents a conceptual term for a describable class of experiences.) Here, 
too, the question is answered by establishing that in conjunction with certain cir- 
cumstances (experimental conditions, concentration of attention, etc.) an experience 
with certain describable properties occurs. Such describable properties wdtild be, for 
example: similarity to an experience that has occurred under certain other circum- 
stances; tendency to evoke certain reactions; and so on, 

However we may twist and tum, it is impossible to interpret a reality-statement 
otherwise than as fitting into a perceptual context. It is absolutely the satne kind of 
reality that we have to attribute to the data of consciousness and to physical events. 
Scarcely anything in the history of philosophy has created more confusion than the 
attempt to pick out one of the two as true 'being'. Wherever the term 'real' is 
intelligibly used, it has one and the same meaning. 

Our Opponent, perhaps, will still feel his position unshaken by what we have said, 
having the impression, rather, that the arguments here presented presuppose a starting- 
point at which he cannot, from the outset, Station himself. He has to concede that the 
decision about the reality or unreality of anything in experience takes place, in every 
case, in the manner outlined, but he claims that in this way we only arrive at what Kant 
called empirical reality. It designates the area govemed by the observations of daily life 
and of science, but beyond this boundary there lies something eise, transcendent reality, 
which caruiot be inferred by strict logic, and is thus no postulate of the understanding, 
though it is a postulate of sound reason. It is the only true extemal world, and this alone 
is at issue in the philosophical problem of the existence of the extemal world. The 
discussion thereupon abandons the question about the meaning of the term 'reality', 
and tums to that about the meaning of the term 'extemal world'. 

The term 'extemal world' is obviously used in two different ways: firstly in the usage 
of daily life, and secondly as a technical term in philosophy. 

Where it occurs in everyday life, it has, like the majority of expressions employed 
in practical affairs, an intelligibly stateable meaning. In contrast to the 'intemal world', 
which Covers memories, thoughts, dreams, wishes and feelings, the 'extemal world' 
means nothing eise, here, but the world of mountains and trees, houses, animals and 



50 Moritz Schlick 

men. What it means to maintain the existence of a certain object in this world, is 
known to every child; and it was necessary to point out that it really means absolutely 
nothing more than what the child knows. We all know how to verify the proposition, 
say, that There is a Castle in the park before the town'. We perform certain acts, and 
if certain exactly specifiable states-of-affairs come about, then we say: Tes, there really 
is a Castle there'; otherwise we say: That statement was an error or a lie/ And if 
somebody now asks us: 'But was the Castle there in the night as well, when nobody 
saw it?' we answer: 'Undoubtedly! for it would have been impossible to build it in the 
period from early this moming tili now, and besides, the state of the building shows 
that it was not only already in situ yesterday, but has been there for a hundred years, 
and hence since before we were bom'. We are thus in possession of quite specific 
empirical criteria for whether houses and trees were also there when we were not 
seeing them, and whether they already existed before our birth, and will exist after 
our death, That is to say, the claim that these things 'exist independently of us' has a 
perfectly clear, testable meaning, and is obviously to be answered in the affirmative. 
We are very well able to distinguish such things in a stateable way from those that 
only occur 'subjectively', 'in dependence upon ourselves'. If, owing to an eye defect, I 
see, for example, a dark speck when I look at the wall opposite me, 1 say of it that it is 
there only when I look, whereas I say of the wall that it is also there when I am not 
looking. The verification of this difference is in fact very easy, and both claims assert 
precisely what is contained in these verifications and nothing more. 

So if the term extemal world' is taken in the everyday sense, the question about its 
existence simply means: Are there, in addition to memories, wishes and ideas, also 
Stars, clouds, plants and animals, and my own body? We have just affirmed once more 
that it would be utterly absurd to say no to this question. There are obviously houses 
and clouds and animals existing independently of us, and I have already said earlier 
that a thinker who denied the existence of the extemal world in this sense would have 
no claim to our attention. Instead of telling us what we mean when we speak of 
mountains and plants, he wishes to persuade us that there are no such things at all! 

But now how about science? When it speaks of the extemal world, does it, unlike 
daily life, mean something other than things such as houses and trees? It seems to me 
that this is by no means the case. For atoms and electric fields, or whatever eise the 
physicist may speak of, are precisely what houses and trees consist of, according to his 
teaching; the one must therefore be real in the same sense as the other, The objectivity 
of mountains and clouds is just exactly the same as that of protons and energies; the 
latter stand in no greater contrast to the 'subjectivity' of feelings, say, or hallucinations, 
than do the former. We have long since convinced ourselves, in fact, that the existence 
of even the most subtle of the 'invisible' things postulated by the scientist is verified, 
in principle, in exactly the same way as the reality of a tree or a star. 

In Order to settle the dispute about realism, it is of the greatest importance to alert 
the physicist to the fact that his extemal world is nothing eise but the nature which also 
surrounds us in daily life, and is not the 'transcendent world' of the metaphysicians. 
The difference between the two is again quite particularly evident in the philosophy of 
Kant. Nahire, and everything of which the physicist can and must speak, belongs, in 
Kant's view, to empirical reality, and the meaning of this (as already mentioned) is 
explained by him exactly as we have also had to do. Atoms, in Kant's System, have no 
transcendent reality — they are not 'things-in-themselves'. Thus the physicist cannot 
appeal to the Kantian philosophy; his arguments lead only to the empirical extemal 
world that we all acknowledge, not to a transcendent one; his electrons are not 
metaphysical entities. 



Positivism and Realism 51 

Many scientists speak, nonetheless, of the necessity of having to postulate the exis- 
tence of an extemal world as a meiaphysical hypothesis. They never do this, indeed, 
within their own science (although all the necessary hypotheses of a science ought to 
occur within it), but only at the point where they leave this territory and begin to 
philosophize. The transcendent extemal world is actually something that is referred to 
exclusively in philosophy, never in a science or in daily life. It is simply a technical 
term, whose meaning we now have to inquire into. 

How does the transcendent or metaphysical extemal world differ from the empirical 
one? In philosophical Systems it is thought of as subsisting somehow behind the 
empirical world, where the word 'behind' is also supposed to indicate that this world 
is not knowable in the same sense as the empirical, that it lies beyond a boundary that 
divides the accessible from the inaccessible. 

This distinction originally has its ground in the view fomierly shared by the 
majori ty of philosophers, that to know an object requires that it be immediately 
givea directly experienced; knowledge is a kind of intuition, and is perfect only if the 
known is directly present to the knower, like a Sensation or a feeling. So what cannot 
be immediately experienced or intuited remains, on this view, unknowable, ungrasp- 
able, transcendent, and belongs to the realm of things-in-themselves. Here, as I have 
elsewhere had to state on numerous occasions, we simply have a confusion of knowing 
with mere acquaintance or experiencing. But such a confusion is certainly not com- 
mitted by modern scientists; I do not believe that any physicist considers knowledge 
of the electron to consist in its entering bodily, by an act of intuition, into the 
scientist's consciousness; he will take the view, rather, that for complete knowledge the 
only thing needed is for the regularity of an electron' s behaviour to be so exhaustively 
stated that all formulae in which its properties occur in any way are totally confirmed 
by experience. In other words, the electron, and all physical realities likewise, are not 
unknowable things-in-themselves, and do not belong to a transcendent, metaphysical 
reality, if this is characterized by the fact that it embraces the unknowable. 

Thus we again retum to the conclusion that all the physicist' s hypotheses can relate 
only to empirical reality, if by this we mean the knowable. It would in fact be a 
self-contradiction to wish to assume something unknowable as a hypothesis. For 
there must always be specific reasons for setting up a hypothesis, since it is, after all, 
supposed to fulfil a specific purpose. What is assumed in the hypothesis must therefore 
have the property of fulfiUing this purpose, and of being precisely so constituted as to 
be justified by these reasons. But in virtue of this very fact certain statements are made 
of it, and these contain knowledge of it. And they contain, indeed, complete knowledge 
of it, since only that can be hypothetically assumed for which there are reasons in 
experience. 

Or does the scientific 'realist' wish to characterize the talk of not immediately 
experienced objects as a metaphysical hypothesis for some reason other than the 
nonexistent one of its unknowability? To this, perhaps, he will ans wer 'yes'. In fact it 
can be seen from numerous statements in the literature, that the physicist by no means 
couples his claim of a transcendent world with the claim that it is unknowable; on the 
contrary, he (quite rightly) takes the view that the nature of extra-mental things is 
reflected with perfect correctness in his equations. Hence the extemal world of the 
physical realist is not that of traditional metaphysics. He employs the technical term of 
the philosophers, but what he designates by means of it has seemed to us to be merely 
the extemal world of everyday life, whose existence is doubted by nobody, not even 
the 'positivist'. 



52 Moritz Schlick 

So what is this other reason that leads the Vealist' to regard his extemal world as a 
metaphysical assumption? Why does he want to distinguish it from the empirical 
extemal world that we have described? The answer to this question leads us back again 
to an earlier point in our argument. For the Vealistic' physicist is perfectly content with 
our description of the extemal world, except on one point: he thinks that we have not 
lent it enough reality. It is not by its unknowability or any other feature that he takes 
his 'extemal world' to differ from the empirical one; it is simply and solely by the fact 
that another, higher reality attaches to it. This often finds expression even in the 
terminology; the word real' is often reserved for this extemal world, in contrast to the 
merely 'ideal', 'subjective' content of consciousness, and the mere 'logical constmc- 
tions' into which positivism' is accused of dissolving reality. 

But now even the physical realist has a dim feeling that, as we know, reality is not 
a 'property'; hence he cannot simply pass from our empirical extemal world to his 
transcendent one by attributing to it the feature of 'reality' over and above the features 
that we, too, ascribe to all physical objects; yet that is how he talks, and this illegiti- 
mate leap, whereby he leaves the realm of the meaningful, would in fact be 'metaphysi- 
cal', and is also feit to be such by himself. 

We now have a clear view of the Situation, and can judge it on the basis of the 
preceding considerations. 

Our principle, that the truth and falsity of all statements, including those about the 
reality of a physical object, can be tested only in the 'given', and that therefore the 
meaning of all statements can likewise be formulated and understood only by means 
of the given — this principle has been wrongly constmed as if it claimed or pre- 
supposed that only the given is real. Hence the 'realist' feels compelled to contradict 
the principle, and to set up the counterclaim, that the meaning of a reality-statement is 
by no means exhausted in mere assertions of the form 'Under these particular circum- 
stances this particular experience will occur' (where these assertions, on our view, are 
in any case an infinite multitude); the meaning, he says, in fact lies beyond this in 
something eise, which must be referred to, say, as 'independent existence', 'transcen- 
dent being' or the like, and of which our principle provides no account. 

To this we ask: Well, then, how does one give an account of it? What do these 
words 'independent existence' and 'transcendent being' mean? In other words, what 
testable difference does it make in the world, whether an object has transcendent being 
or not? 

Two answers are given here. The first runs: It makes a quite enormous difference. 
For a scientist who believes in a 'real extemal world' will feel and work quite differently 
from one who merely aims at 'describing sensations'. The former will regard the starry 
heaven, whose aspect recalls to him the inconceivable sublimity and size of the 
universe, and his own human smallness, with feelings of awe and devotion quite 
different from those of the latter, to whom the most distant galactic Systems are but 
'complexes of his own sensations' The first will be devoted to his task with an en- 
thusiasm, and will feel in his knowing of the objective world a satisfaction, that are 
denied to the second, since he takes himself to be concemed only with constmctions 
of his own. 

To this first answer we have this to say: If, in the behaviour of two thinkers, there 
should anywhere occur a difference such as has here been described — and it would in 
fact involve an observable state-of-affairs — and were we to insist upon so expressing 
this difference as to say that the first believes in a real extemal world, and the other 
not — well, even so, the meaning of our assertion still consists solely in what we 
observe in the behavior of the two. That is to say, the words 'absolute reality', or 



Positivism and Realism 53 

'transcendent being', or whatever other terms we may use for it, now signify absolutely 
nothing eise but certain states of feeling which arise in the two whenever they 
contemplate the universe, or make reality-statements, or philosophize. The fact of the 
matter is, that employment of the words 'independent existence', 'transcendent reality' 
and so on, is simply and solely the expression of a feeling, a psychological attitude of 
the Speaker (which may in the end, moreover, apply to all metaphysical propositions), 
If someone assures us that there is a real extemal world in the supra-empirical sense of 
the term, he thinks, no doubt, that he has thereby conveyed a truth about the world; 
but in actuality his words express a quite different state-of-affairs, namely the mere 
presence of certain feelings, which provoke him to specific reactions of a verbal or 
other nature. 

If the self-evident still needs to be specially dwelt on, I should like to underline — 
but in that case with maximum emphasis, and with stress upon the seriousness of what 
I am saying — that the nonmetaphysician does not differ from the metaphysician by 
the fact, say, that he lacks those feelings to which the other gives expression by 
way of the propositions of a 'realistic' philosophy, but only by the fact that he has 
recognized that these propositions by no means have the meaning that they seem to 
have, and are therefore to be avoided. He will give expression to the same feelings in 
a different way. In other words, this confrontation of the two types of thinker, set up 
in the realist's' first ans wer, was misleading and erroneous. If anyone is so unfortunate 
as not to feel the sublimity of the starry heaven, then the blame lies on something 
other than a logical analysis of the concepts of reality and the extemal world. To 
suppose that the Opponent of metaphysics is incapable, say, of justly estimating the 
greatness of Copemicus, because in a certain sense the Ptolemaic view reflects the 
empirical Situation just as well as the Copemican, seems to me no less stränge than to 
believe that the 'positivist' cannot be a good father to his family, because according to 
his theory his children are merely complexes of his own sensations, and it is therefore 
senseless to make provision for their welfare after his death. No, the world of the 
non-metaphysician is the same world as that of everybody eise; it lacks nothing that is 
needed in order to make meaningful all the statements of science and all the actions of 
daily life. He merely refuses to add meaningless statements to his description of the 
world. 

We come to the second answer that can be given to the question about the meaning 
of the claim that there is a transcendent reality. It simply consists in admitting that it 
makes absolutely no difference for experience whether we postulate something eise 
existing behind the empirical world or not; metaphysical realism cannot therefore be 
actually tested or verified. Thus it cannot be further stated what is meant by this claim; 
yet something is meant thereby, and the meaning can also be understood without 
verification. 

This is nothing eise but the view criticized in the previous Section, that the meaning 
of a proposition has nothing to do with its verification, and it only remains for us to 
repeat once more our earlier general criticism, as applied to this particular Case. We 
must reply, therefore: Well now! You are giving the name 'existence' or 'reality' here 
to something that is utterly inexpressible and cannot be explained or stated in any 
fashion. You think, nonetheless, that these words have a meaning. As to that, we shall 
not quarrel with you. But this much is certain: by the admission just made, this 
meaning cannot in any way become manifest, cannot be expressed by any oral or 
written communication, or by any gesture or act. For if this were possible, a testable 
empirical Situation would exist; there would be something different in the world, if the 



54 Moritz Schlick 

proposition There is a transcendent world' were true, from if it were false. This 
differentness would then signify the meaning of the words 'real extemal world\ and 
hence it would be an empirical meaning — that is, this real extemal world would again 
be merely the empirical world which we, too, acknowledge, like everyone eise. Even 
to speak, merely, of another world, is logically impossible. There can be no discussion 
about it, for a nonverifiable existence cannot enter as meaning into any possible 
proposition. Anyone who still believes in such a thing — or imagines he believes — can 
only do so in silence, There are arguments only for something that can be said. 
The results of our discussion can be summarized as foUows: 

1. The principle, that the meaning of every proposition is exhaustively de- 
termined by its verification in the given, seems to me a legitimate, unassailable 
core of the positivist' schools of thought. 

But within these schools it has seldom come clearly to light, and has often 
been mingled with so many untenable principles, that a logical clean-up is 
necessary. If we want to call the result of this clean-up positivism', which might 
well be justified on historical grounds, we should have, perhaps, to affix a 
difFerentiating adjective: the term"* logical' or 'logistic positivism' is often used; 
otherwise the expression consistent empiricism' has seemed to me appropriate. 

2. This principle does not mean, nor does it foUow from it, that only the given 
is real; such a claim would actually be meaningless. 

3. Consistent empiricism, therefore, does not deny, either, the existence of an 
extemal world; it merely points out the empirical meaning of this existence- 
claim. 

4. It is not an as if theory'. It does not say, for example, that everything behaves 
as if there were physical independent bodies; on the contrary, for it, too, every- 
thing is real that the nonphilosophizing scientist declares to be real. The sub- 
ject matter of physics does not consist of sensations, but of laws. The formulation 
employed by some positivists, that bodies are mere complexes of sensations' is 
therefore to be rejected. The only correct view is that propositions about bodies 
can be transformed into propositions of like meaning about the regularity of 
occurrence of sensations.^ 

5. Logical positivism and realism are therefore not opposed; anyone who ac- 
knowledges our principle must actually be an empirical realist. 

6. There is Opposition only between consistent empiricism and the metaphysi- 
cian, and it is directed as much against the realist as the Idealist (the former is 
designated in our discussion as a 'realist', in quotation-marks). 

7. The denial of the existence of a transcendent extemal world would be just as 
much a metaphysical proposition as its assertion; the consistent empiricist does 
not therefore deny the transcendent, but declares both its denial and its affimma- 
tion to be equally devoid of meaning. 

This last distinction is of the greatest importance. I am convinced that the main 
resistances to our viewpoint stem from the fact that the difference between the 
falsity and the meaninglessness of a proposition is not heeded. The proposition 
Talk of a metaphysical extemal world is meaningless' does not say 'There is no 
metaphysical extemal world', but something toto coelo different. The empiricist does 
not say to the metaphysician: 'Your words assert something false', but Tour words 
assert nothing at all!' He does not contradict the metaphysician, but says: 'I do not 
understand you'. 



Positivism and Realism 55 



Notes 



1. [E, Laas, Idealismus und Positivismus, Eine kritische Äuseinanderseizung. Berlin 1879-1681.] 

2. M. Planck, Positivismus und reale Aussenwelt, Leipzig 1931, p. 14. 

3. a.ibid.,p. 11, 

4. Cf. the article by A. E. Blumberg and H. Feigl ["Logical Positivism"] in The Journal of Philosophy 28 
(1931); See also E. Kaila ['Der logistische Neupositinsmus. Eine kritische Studie'] in Annales Universiiatis 
Äboensis 13 (1930), and A. Petzall ("Logistischer Positivismus"] in Göreborgs Högskolas Arsskrift 37 
(1931). 

5. On this, as on the content of the whole essay, cf. the article by H. Cornelius ['Zur Kritik der 
wissenschaftlichen Grundbegriffe'] in Erkenntnis 2 (1931). The formulations there are admittedly not 
free from objection. Cf. also the outstanding discussion by Philipp Frank in chapter X of his book Das 
Kausalgesetz und seine Grenzen, Wien 1932, and Rudolf Camap, Scheinprobleme in der Philosophie, Leipzig 
and Berlin 1928.