18	THE  FACTORS OF THE MIND
chemistry as well—what we are really analysing are not
substances, but properties : and by properties (as will be
seen later on) I understand not attributes inherent in sub-
stances, but simply relations manifested under certain con-
stant or standard conditions. Thus, the ^c verbal factor' is
not necessarily to be identified with a * verbal centre ' in the
brain or a ^c verbal faculty ' in the mind, any more than
* acidity ' is to be identified with a special chemical substance
or a special chemical force. It would be better, therefore,
to seek some other analogy.
Nor is it difficult to find one, for every science exploits
the same procedure. To take an example familiar to every
schoolboy, we may compare the advantages of using indepen-
dent factors in psychology with those of using latitude and
longitude in geography, where, instead of stating that such
and such a place lies so many miles or kilometres in such a
direction from the place at which the speaker happens at
the moment to be standing, we say that it is so many degrees
north or south of one arbitrary line, and so many degrees
east or west of another arbitrary line drawn at right angles
to the first. There is no visible ^c line ' to be crossed at the
equator ; there are no concrete * poles' to distinguish the
northerly or the southerly directions from all the rest. Yet
the unreality of the lines and points that are marked down
upon our maps does not destroy their utility. In the same
way, to disprove the concrete existence of a psychological
factor is not—as is so often supposed—-to abolish its scientific
value or validity.
Our factors, therefore, are to be thought of in the first
instance as lines or terms of reference only, not as concrete
psychological entities. In order to give an adequate descrip-
tion of persons we must first discover in what independent
directions a person may vary, and, at the same time, so far
as possible, choose the direction so that each may carry with
it a maximum amount of dependent variation*
The early descriptions of plants in the ancient herbals simply
seized on their most conspicuous, if superficial, modes of variation :
the size of the specimen, and the colour and the shape of its flowers,
Here are three directions, apparently uncorrelated, in which a plant
can vary, the variation in one respect being independent of variation