ciation work
in either direction, or only from the one which has occurred earlier
to the one which has occurred later? In an article by Mr. Wohlgemuth,
called "The Direction of Associations" ("British Journal of Psychology,"
vol. v, part iv, March, 1913), it is claimed to be proved by experiment
that, in so far as motor memory (i.e. memory of movements) is concerned,
association works only from earlier to later, while in visual and
auditory memory this is not the case, but the later of two neighbouring
experiences may recall the earlier as well as the earlier the later.
It is suggested that motor memory is physiological, while visual and
auditory memory are more truly psychological. But that is not the point
which concerns us in the illustration. The point which concerns us
is that a law of association, established by purely psychological
observation, is a purely psychological law, and may serve as a sample
of what is possible in the way of discovering such laws. It is, however,
still no more than a rough generalization, a statistical average. It
cannot tell us what will result from a given cause on a given occasion.
It is a law of tendency, not a precise and invariable law such as those
of physics aim at being.

If we wish to pass from the law of habit, stated as a tendency or
average, to something more precise and invariable, we seem driven to the
nervous system. We can more or less guess how an occurrence produces a
change in the brain, and how its repetition gradually produces something
analogous to the channel of a river, along which currents flow more
easily than in neighbouring paths. We can perceive that in this way, if
we had more knowledge, the tendency to habit through repetition might
be replaced by a precise account of the effect of each occurrence
in bringing about a modification of the sort from which habit would
ultimately result. It is such considerations that make students of
psychophysiology materialistic in their methods, whatever they may be in
their metaphysics. There are, of course, exceptions, such as Professor
J. S. Haldane,* who maintains that it is theoretically impossible to
obtain physiological explanations of psychical phenomena, or physical
explanations of physiological phenomena. But I think the bulk of expert
opinion, in practice, is on the other side.

*See his book, "The New Physiology and Other Addresses"
(Charles Griffin & Co., 1919).

The question whether it is possible