HAS CAUSED A COMPLEX REACTION B IN AN ORGANISM,
THE OCCURRENCE OF A PART OF A ON A FUTURE OCCASION TENDS TO CAUSE THE
WHOLE REACTION B.

This law would need to be supplemented by some account of the influence
of frequency, and so on; but it seems to contain the essential
characteristic of mnemic phenomena, without admixture of anything
hypothetical.

Whenever the effect resulting from a stimulus to an organism differs
according to the past history of the organism, without our being able
actually to detect any relevant difference in its present structure,
we will speak of "mnemic causation," provided we can discover laws
embodying the influence of the past. In ordinary physical causation,
as it appears to common sense, we have approximate uniformities of
sequence, such as "lightning is followed by thunder," "drunkenness
is followed by headache," and so on. None of these sequences are
theoretically invariable, since something may intervene to disturb
them. In order to obtain invariable physical laws, we have to proceed to
differential equations, showing the direction of change at each moment,
not the integral change after a finite interval, however short. But
for the purposes of daily life many sequences are to all in tents and
purposes invariable. With the behaviour of human beings, however, this
is by no means the case. If you say to an Englishman, "You have a smut
on your nose," he will proceed to remove it, but there will be no such
effect if you say the same thing to a Frenchman who knows no English.
The effect of words upon the hearer is a mnemic phenomena, since it
depends upon the past experience which gave him understanding of the
words. If there are to be purely psychological causal laws, taking no
account of the brain and the rest of the body, they will have to be of
the form, not "X now causes Y now," but--

"A, B, C,... in the past, together with X now, cause Y now." For it
cannot be successfully maintained that our understanding of a word, for
example, is an actual existent content of the mind at times when we
are not thinking of the word. It is merely what may be called a
"disposition," i.e. it is capable of being aroused whenever we hear the
word or happen to think of it. A "disposition" is not something actual,
but merely the mnemic portion of a mnemic causal law.

In such a law as "A, B, C,... in the past, together with X now, cause
Y now," we will call A, B, C,... the mnemic cause, X the occasion or