antecedents in the brain and
senses and determinate consequents in actions and words. But this
dependence and this efficacy have nothing logical about them; they are
habitual collocations in the world, like lightning and thunder. A more
minute inspection of psycho-physical processes, were it practicable,
would doubtless disclose undreamed of complexities and harmonies in
them; the mathematical and dynamic relations of stimulus and sensation
might perhaps be formulated with precision. But the terms used in the
equation, their quality and inward habit, would always remain data which
the naturalist would have to assume after having learned them by
inspection. Movement could never be deduced dialectically or graphically
from thought nor thought from movement. Indeed no natural relation is in
a different case. Neither gravity, nor chemical reaction, nor life and
reproduction, nor time, space, and motion themselves are logically
deducible, nor intelligible in terms of their limits. The phenomena have
to be accepted at their face value and allowed to retain a certain
empirical complexity; otherwise the seed of all science is sterilised
and calculation cannot proceed for want of discernible and pregnant
elements.

How fine nature's habits may be, where repetition begins, and down to
what depth a mathematical treatment can penetrate, is a question for
the natural sciences to solve. Whether consciousness, for instance,
accompanies vegetative life, or even all motion, is a point to be
decided solely by empirical analogy. When the exact physical conditions
of thought are discovered in man, we may infer how far thought is
diffused through the universe, for it will be coextensive with the
conditions it will have been shown to have. Now, in a very rough way, we
know already what these conditions are. They are first the existence of
an organic body and then its possession of adaptable instincts, of
instincts that can be modified by experience. This capacity is what an
observer calls intelligence; docility is the observable half of reason.
When an animal winces at a blow and readjusts his pose, we say he feels;
and we say he thinks when we see him brooding over his impressions, and
find him launching into a new course of action after a silent decoction
of his potential impulses. Conversely, when observation covers both the
mental and the physical process, that is, in our own experience, we find
that felt impulses, the conceived objects