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 alone which renders our experience useful to us, and makes us expect, for the future, a similar train of events with those which have appeared in the past."...
"All belief of matter-of-fact or real existence is derived merely from some object present to the memory or senses, and a customary conjunction between that and some other object; or in other words, having found, in many instances, that any two kinds of objects, flame and heat, snow and cold, have always been conjoined together: if flame or snow be presented anew to the senses, the mind is carried by custom to expect heat or cold, and to _believe_ that such a quality does exist, and will discover itself upon a nearer approach.

This belief is the necessary result of placing the mind in such circumstances.

It is an operation of the soul, when we are so situated, as unavoidable as to feel the passion of love, when we receive benefits, or hatred, when we meet with injuries.

All these operations are a species of natural instincts, which no reasoning or process of the thought and understanding is able either to produce or to prevent."-- (IV.pp.