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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 stimulus, and Y the reaction. All cases in
which experience influences behaviour are instances of mnemic
causation.
Believers in psychophysical 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
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 dead matter. The argument from the existence of mnemic
phenomena in physiology must therefore be allowed a certain
weight against the hypothesis that mnemic causation is ultimate.
* See his “The New Physiology and Other Addresses,” Griffin,
1919, also the symposium, “Are Physical, Biological and
Psychological Categories Irreducible?” in “Life and Finite
Individuality,” edited for the Aristotelian Society, with an
Introduction. By H. Wildon Carr, Williams & Norgate, 1918.
The argument from the connection of brain-lesions with loss of
memory is not so strong as it looks, though it has also, some
weight. What we know is that memory, and mnemic phenomena
generally, can be disturbed or destroyed by changes in the brain.
This certainly proves that the brain plays an essential part in
the causation of memory, but does not prove that a certain state
of the brain is, by itself, a sufficient condition for the
existence of memory. Yet it is this last that has to be proved.
The theory of the engram, or any similar theory, has to maintain
that, given a body and brain in a suitable state, a man will have
a certain memory, without the need of any further conditions.
What is known, however, is only that he will not have memories if
his body and brain are not in a suitable state. That is to say,
the appropriate state of body and brain is proved to be necessary
for memory, but not to be sufficient. So far, therefore, as our
definite knowledge goes, memory may require for its causation a
past occurrence as well as a certain present state of the brain.
In order to prove conclusively that mnemic phenomena arise
whenever certain physiological conditions are fulfilled, we ought
to be able actually to see differences between the brain of a man
who speaks English and that of a man who speaks French, between
the brain of a man who has seen New York and can recall it, and
that of a man who has never seen that city. It may be that the
time will come when this will be possible, but at present we are
very far removed from it. At present, there is, so far as I am
aware, no good evidence that every difference between the
knowledge possessed by A and that possessed by B is paralleled by
some difference in their brains. We may believe that this is the
case, but if we do, our belief is based upon analogies and
general scientific maxims, not upon any foundation of detailed
observation. I am myself inclined, as a working hypothesis, to
adopt the belief in question, and to hold that past experience
only affects present behaviour through modifications of
physiological structure. But the evidence seems not quite
conclusive, so that I do not think we ought to forget the other
hypothesis, or to reject entirely the possibility that mnemic
causation may be the ultimate explanation of mnemic phenomena. I
say this, not because I think it LIKELY that mnemic causation is
ultimate, but merely because I think it POSSIBLE, and because it
often turns out important to the progress of science to remember
hypotheses which have previously seemed improbable.
LECTURE V. PSYCHOLOGICAL AND PHYSICAL CAUSAL LAWS
The traditional conception of cause and effect is one which
modern science shows to be fundamentally erroneous, and requiring
to be replaced by a quite different notion, that of LAWS OF
CHANGE. In the traditional conception, a particular event A
caused a particular event B, and by this it was implied that,
given any event B, some earlier event A could be discovered which
had a relation to it, such that—
(1) Whenever A occurred, it was followed by B;
(2) In this sequence, there was something “necessary,” not a mere
de facto occurrence of A first and then B.
The second point is illustrated by the old discussion as to
whether it can be said that day causes night, on the ground that
day is always followed by night. The orthodox answer was that day
could not be called the cause of night, because it would not be
followed by night if the earth’s rotation were to cease, or
rather to grow so slow that one complete rotation would take a
year. A cause, it was held, must be such that under no
conceivable circumstances could it fail to be followed by its
effect.
As a matter of fact, such sequences as were sought by believers
in the traditional form of causation have not so far been
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