to it, in order to account for the
mistakes that arise from these relations. I shall therefore
observe, that as the mind is endowed with the power of exciting any
idea it pleases; whenever it despatches the spirits into that
region of the brain in which the idea is placed; these spirits
always excite the idea, when they run precisely into the proper
traces and rummage that cell which belongs to the idea. But as
their motion is seldom direct, and naturally turns a little to the
one side or to the other; for this reason the animal spirits,
falling into the contiguous traces, present other related ideas, in
lieu of that which the mind desired at first to survey. This change
we are not always sensible of; but continuing still the same train
of thought, make use of the related idea which is presented to us
and employ it in our reasonings, as if it were the same with what
we demanded. This is the cause of many mistakes and sophisms in
philosophy; as will naturally be imagined, and as it would be easy
to show, if there was occasion."--(I. p. 88.)

Perhaps it is as well for Hume's fame that the occasion for further
physiological speculations of this sort did not arise. But, while
admitting the crudity of his notions and the strangeness of the language
in which they are couched, it must in justice be remembered, that what
are now known as the elements of the physiology of the nervous system
were hardly dreamed of in the first half of the eighteenth century; and,
as a further set off to Hume's credit, it must be noted that he grasped
the fundamental truth, that the key to the comprehension of mental
operations lies in the study of the molecular changes of the nervous
apparatus by which they are originated.

Surely no one who is cognisant of the facts of the case, nowadays,
doubts that the roots of psychology lie in the physiology of the nervous
system. What we call the operations of the mind are functions of the
brain, and the materials of consciousness are products of cerebral
activity. Cabanis may have made use of crude and misleading phraseology
when he said that the brain secretes thought as the liver secretes bile;
but the conception which that much-abused phrase embodies is,
nevertheless, far more consistent with fact than the popular notion that
the mind is a metaphysical entity seated in the head, but as independent
of the brain as a telegra