Memory vanishes with extremes of resemblance or difference. Things which
put us in mind of others must be neither too like nor too unlike them.
It is our sense that a position is not quite the same which makes us find
it so nearly the same. We remember by the aid of differences as much as
by that of samenesses. If there could be no difference there would be no
memory, for the two positions would become absolutely one and the same,
and the universe would repeat itself for ever and ever as between these
two points.
When ninety-nine hundredths of one set of phenomena are presented while
the hundredth is withdrawn without apparent cause, so that we can no
longer do something which according to our past experience we ought to
find no difficulty in doing, then we may guess what a bee must feel as it
goes flying up and down a window-pane. Then we have doubts thrown upon
the fundamental axiom of life, i.e. that like antecedents will be
followed by like consequents. On this we go mad and die in a short time.
Mistaken memory may be as potent as genuine recollection, so far as its
effects go, unless it happens to come more into collision with other and
not mistaken memories than it is able to contend against.
Mistakes or delusions occur mainly in two ways.
First, when the circumstances have changed a little but not enough to
make us recognise the fact: this may happen either because of want of
attention on our part or because of the hidden nature of the alteration,
or because of its slightness in itself, the importance depending upon its
relations to something else which make a very small change have an
importance it would not otherwise have: in these cases the memory reverts
to the old circumstances unmodified, a sufficient number of the
associated ideas having been reproduced to make us assume the remainder
without further inspection, and hence follows a want of harmony between
action and circumstances which results in trouble somewhere.
Secondly, through the memory not reverting in full perfection, though the
circumstances are reproduced fully and accurately.