tion. Nay, 'tis not only possible we may
have such an experience, but 'tis certain we have it; since every
one may perceive that the different dispositions of his body change
his thoughts and sentiments. And should it be said that this
depends on the union of soul and body, I would answer, that we must
separate the question concerning the substance of the mind from
that concerning the cause of its thought; and that, confining
ourselves to the latter question, we find, by the comparing their
ideas, that thought and motion are different from each other and by
experience, that they are constantly united; which, being all the
circumstances that enter into the idea of cause and effect, when
applied to the operations of matter, we may certainly conclude that
motion may be, and actually is, the cause of thought and
perception."--(I. pp. 314-316.)

The upshot of all this is, that the "collection of perceptions," which
constitutes the mind, is really a system of effects, the causes of which
are to be sought in antecedent changes of the matter of the brain, just
as the "collection of motions," which we call flying, is a system of
effects, the causes of which are to be sought in the modes of motion of
the matter of the muscles of the wings.

Hume, however, treats of this important topic only incidentally. He
seems to have had very little acquaintance even with such physiology as
was current in his time. At least, the only passage of his works,
bearing on this subject, with which I am acquainted, contains nothing
but a very odd version of the physiological views of Descartes:--

"When I received the relations of _resemblance_, _contiguity_, and
_causation_, as principles of union among ideas, without examining
into their causes, 'twas more in prosecution of my first maxim,
that we must in the end rest contented with experience, than for
want of something specious and plausible which I might have
displayed on that subject. 'Twould have been easy to have made an
imaginary dissection of the brain, and have shown why, upon our
conception of any idea, the animal spirits run into all the
contiguous traces and rouse up the other ideas that are related to
it. But though I have neglected any advantage which I might have
drawn from this topic in explaining the relations of ideas, I am
afraid I must here have recourse