METAPHYSICAL STATUS OF FACTORS 215

I think, if we insisted that the temperature of the gas
had no real existence, or warned him not to reify heat,
and argued that the only real entities must be the moving
molecules, which, as we had conceived them, were neverthe-
less quite unlike any actual molecules. Yet, from a severely
philosophical standpoint, all these strictures would be fully
warranted ; and we could safely add that the quantity called
heat appeared simply as a solution to an algebraic equation,
and not as a fund of energy that could be observed at first
hand.

The student, reading Thomson's argument, takes the gist
of it to be that Spearman's g cannot exist in the sense in
which physical energy exists; and his impression seems
confirmed when Spearman himself admits that the identifi-
cation of g with some such energy is only a tentative sub-
theory, and not essential to his main hypothesis. Both
parties appear to overlook the fact that, if the truth be
strictly told, even physical energy can claim no real existence,
It is, indeed, a glaring instance of the faculty fallacy. We
feel warmth, see light, hear sound, experience movement
and resistance. To account for such effects the nineteenth-
century physicist simply postulated an c energy ' of heat, of
light, of sound ; to explain the fact that a moving body does
work he simply endowed the body with a c capacity' for
such work ; worse still, when the energy does not manifest
itself visibly to the senses, he invented yet another capacity
that he called ' potential' energy. Then, at a later stage
of his argument, he declared, in terms like Spearman's, that
there are not a number of such capacities, all specific and
irreducibly distinct; there is just a single general capacity,
common to all; the various forms of energy, which we
originally classified according to the sensory experiences they
apparently excite, are to be regarded as manifestations of
one and the same indestructible energy, which is pictured
as fundamentally kinetic.

No doubt, the physicist of to-day would be far less con-
fident about the real existence of the energy thus postulated.
He would probably declare that the question was irrelevant
to his line of thought. The assumption of such a concept
enables him to unify his science, to relate his various