repetition of any particular act or operation
produces a propensity to renew the same act or operation, without
being impelled by any reasoning or process of the understanding, we
always say that this propensity is the effect of _Custom_. By
employing that word, we pretend not to have given the ultimate
reason of such a propensity. We only point out a principle of human
nature which is universally acknowledged, and which is well known
by its effects."--(IV. p. 52.)

It has been shown that an expectation is a complex idea which, like a
memory, is made up of two constituents. The one is the idea of an
object, the other is the idea of a relation of sequence between that
object and some present object; and the reasoning which applied to
memories applies to expectations. To have an expectation[25] of a given
event, and to believe that it will happen, are only two modes of stating
the same fact. Again, just in the same way as we call a memory, put into
words, a belief, so we give the same name to an expectation in like
clothing. And the fact already cited, that a child before it can speak
acts upon its memories, is good evidence that it forms expectations. The
infant who knows the meaning neither of "sugar-plum" nor of "sweet,"
nevertheless is in full possession of that complex idea, which, when he
has learned to employ language, will take the form of the verbal
proposition, "A sugar-plum will be sweet."

Thus, beliefs of expectation, or at any rate their potentialities, are,
as much as those of memory, antecedent to speech, and are as incapable
of justification by any logical process. In fact, expectations are but
memories inverted. The association which is the foundation of
expectation must exist as a memory before it can play its part. As Hume
says,--

" ... it is certain we here advance a very intelligible proposition
at least, if not a true one, when we assert that after the constant
conjunction of two objects, heat and flame, for instance, weight
and solidity, we are determined by custom alone to expect the one
from the appearance of the other. This hypothesis seems even the
only one which explains the difficulty why we draw from a thousand
instances, an inference which we are not able to draw from one
instance, that is in no respect different from them."...

"Custom, then, is the great guide of human life. It is that
principle alon