object, which is present to my senses, and that other, whose existence I
infer by reasoning, may be thought to influence each other by their
particular powers or qualities; yet as the phenomenon of belief, which we
at present examine, is merely internal, these powers and qualities, being
entirely unknown, can have no hand in producing it. It is the present
impression, which is to be considered as the true and real cause of the
idea, and of the belief which attends it. We must therefore endeavour to
discover by experiments the particular qualities, by which it is enabled
to produce so extraordinary an effect.

First then I observe, that the present impression has not this effect by
its own proper power and efficacy, and when considered alone, as a single
perception, limited to the present moment. I find, that an impression,
from which, on its first appearance, I can draw no conclusion, may
afterwards become the foundation of belief, when I have had experience of
its usual consequences. We must in every case have observed the same
impression in past instances, and have found it to be constantly
conjoined with some other impression. This is confirmed by such a
multitude of experiments, that it admits not of the smallest doubt.

From a second observation I conclude, that the belief, which attends the
present impression, and is produced by a number of past impressions and
conjunctions; that this belief, I say, arises immediately, without any
new operation of the reason or imagination. Of this I can be certain,
because I never am conscious of any such operation, and find nothing in
the subject, on which it can be founded. Now as we call every thing
CUSTOM, which proceeds from a past repetition, without any new reasoning
or conclusion, we-may establish it as a certain truth, that all the
belief, which follows upon any present impression, is derived solely from
that origin. When we are accustomed to see two impressions conjoined