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.
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