stimulus, and Y the reaction. All cases in which experience influences
behaviour are instances of mnemic causation.

Believers in psycho-physical parallelism hold that psychology can
theoretically be freed entirely from all dependence on physiology or
physics. That is to say, they believe that every psychical event has
a psychical cause and a physical concomitant. If there is to be
parallelism, it is easy to prove by mathematical logic that the
causation in physical and psychical matters must be of the same sort,
and it is impossible that mnemic causation should exist in psychology
but not in physics. But if psychology is to be independent of
physiology, and if physiology can be reduced to physics, it would seem
that mnemic causation is essential in psychology. Otherwise we shall be
compelled to believe that all our knowledge, all our store of images
and memories, all our mental habits, are at all times existing in some
latent mental form, and are not merely aroused by the stimuli which lead
to their display. This is a very difficult hypothesis. It seems to me
that if, as a matter of method rather than metaphysics, we desire to
obtain as much independence for psychology as is practically feasible,
we shall do better to accept mnemic causation in psychology protem,
and therefore reject parallelism, since there is no good ground for
admitting mnemic causation in physics.

It is perhaps worth while to observe that mnemic causation is what led
Bergson to deny that there is causation at all in the psychical sphere.
He points out, very truly, that the same stimulus, repeated, does not
have the same consequences, and he argues that this is contrary to the
maxim, "same cause, same effect." It is only necessary, however, to take
account of past occurrences and include them with the cause, in order
to re-establish the maxim, and the possibility of psychological causal
laws. The metaphysical conception of a cause lingers in our manner of
viewing causal laws: we want to be able to FEEL a connection between
cause and effect, and to be able to imagine the cause as "operating."
This makes us unwilling to regard causal laws as MERELY observed
uniformities of sequence; yet that is all that science has to offer.
To ask why such-and-such a kind of sequence occurs is either to ask a
meaningless question, or to demand some more general kind of sequence
which includes the one in question. The widest empirical laws of
sequence known at any