218 THE FACTORS OF THE MIND

Just as we cannot deduce the essential character of the
elementary processes in the retina from a mere analysis of
colour equations, so we cannot determine the nature of
intelligence without supplementary evidence from anatomy,
physiology, and genetics, to be accumulated by research that
is only just beginning. Meanwhile, if (as many factorists
affirm) it is unfair, in our present state of ignorance, to
import physiological considerations into the picture, and if
we are consequently to take our factors as describing, not
hypothetical characteristics of the individual nervous
system, but only observable characteristics of the individual
mind, then certainly we had better refrain altogether from
referring to such factors as concrete entities : to speak of
* factors in the mind ' as if they existed in the same way as,
but in addition to, the physical organs and tissues of the
body and their properties, is assuredly both indefensible and
misleading.

Strictly, therefore, the scientist can never really measure
mental abilities as entities in themselves ; for there is no
ground for believing that such abilities can have any real
existence apart from the behaviour by which they are
displayed or the organism that displays them. What we
call an ability is simply a convenient name for designating
a set of potential reactions on the part of the individual
tested.

(ii) The Causal Efficacy of Factors.—If our mental factors
as such can claim no necessary concrete existence, still less
can we endow them with effective causal powers. Ex-
planation in terms of causal agencies is a legacy of nineteenth-
century science—a mode of approach which should now be
considered as out-of-date in mental science as it is in physical
science. In psychology concepts of this kind appear little
more than relics of the old-fashioned faculties, which were
invented to perform the same function in psychology as
were performed by forces in physics, and have since got
ingrained in our everyday habits of speech. Nearly all
psychologists are nowadays agreed in repudiating mental
faculties, or at any rate the name ; but their reason for this
rejection seems rather that there is little or no objective
evidence for any set of faculties that has hitherto been