time can only be "explained" in the sense of being
subsumed by later discoveries under wider laws; but these wider laws,
until they in turn are subsumed, will remain brute facts, resting solely
upon observation, not upon some supposed inherent rationality.

There is therefore no a priori objection to a causal law in which part
of the cause has ceased to exist. To argue against such a law on the
ground that what is past cannot operate now, is to introduce the old
metaphysical notion of cause, for which science can find no place. The
only reason that could be validly alleged against mnemic causation would
be that, in fact, all the phenomena can be explained without it. They
are explained without it by Semon's "engram," or by any theory which
regards the results of experience as embodied in modifications of
the brain and nerves. But they are not explained, unless with extreme
artificiality, by any theory which regards the latent effects of
experience as psychical rather than physical. Those who desire to make
psychology as far as possible independent of physiology would do well,
it seems to me, if they adopted mnemic causation. For my part, however,
I have no such desire, and I shall therefore endeavour to state the
grounds which occur to me in favour of some such view as that of the
"engram."

One of the first points to be urged is that mnemic phenomena are just
as much to be found in physiology as in psychology. They are even to
be found in plants, as Sir Francis Darwin pointed out (cf. Semon, "Die
Mneme," 2nd edition, p. 28 n.). Habit is a characteristic of the body
at least as much as of the mind. We should, therefore, be compelled
to allow the intrusion of mnemic causation, if admitted at all, into
non-psychological regions, which ought, one feels, to be subject only to
causation of the ordinary physical sort. The fact is that a great deal
of what, at first sight, distinguishes psychology from physics is found,
on examination, to be common to psychology and physiology; this whole
question of the influence of experience is a case in point. Now it
is possible, of course, to take the view advocated by Professor J. S.
Haldane, who contends that physiology is not theoretically reducible to
physics and chemistry.* But the weight of opinion among physiologists
appears to be against him on this point; and we ought certainly to
require very strong evidence before admitting any such breach of
continuity as between living and