In the absence of a distinct nervous system, we have no right to look
for its product, consciousness; and, even in those forms of animal life
in which the nervous apparatus has reached no higher degree of
development, than that exhibited by the system of the spinal cord and
the foundation of the brain in ourselves, the argument from analogy
leaves the assumption of the existence of any form of consciousness
unsupported. With the super-addition of a nervous apparatus
corresponding with the cerebrum in ourselves, it is allowable to suppose
the appearance of the simplest states of consciousness, or the
sensations; and it is conceivable that these may at first exist, without
any power of reproducing them, as memories; and, consequently, without
ideation. Still higher, an apparatus of correlation may be superadded,
until, as all these organs become more developed, the condition of the
highest speechless animals is attained.
It is a remarkable example of Hume's sagacity that he perceived the
importance of a branch of science which, even now, can hardly be said to
exist; and that, in a remarkable passage, he sketches in bold outlines
the chief features of comparative psychology.
" ... any theory, by which we explain the operations of the
understanding, or the origin and connexion of the passions in man,
will acquire additional authority if we find that the same theory
is requisite to explain the same phenomena in all other animals.
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