the chain of natural causes. It is at least a link in the chain of natural events; for it has determinate 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