hesis, 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 found in nature.
Everything in nature is apparently in a state of continuous change,* so
that what we call one "event" turns out to be really a process. If this
event is to cause another event, the two will have to be contiguous in
time; for if there is any interval between them, something may happen
during that interval to prevent the expected effect. Cause and effect,
therefore, will have to be temporally contiguous processes. It is
difficult to believe, at any rate where physical laws are concerned,
that the earlier part of the process which is the cause can make any
dif