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 together, the appearance or idea of the one immediately carries us to the idea of the other.

Being fully satisfyed on this head, I make a third set of experiments, in order to know, whether any thing be requisite, beside the customary transition, towards the production of this phaenomenon of belief.  I therefore change the first impression into an idea; and observe, that though the customary transition to the correlative idea still remains, yet there is in reality no belief nor perswasion.  A present impression, then, is absolutely requisite to this whole operation; and when after this I compare an impression with an idea, and find that their only difference consists in their different degrees of force and vivacity, I conclude upon the whole, that belief is a more vivid and intense conception of an idea, proceeding from its relation to a present impression.

Thus all probable reasoning is nothing but a species of sensation.  It is not solely in poetry and music, we must follow our taste and sentiment, but likewise in philosophy.  When I am convinced of any principle, it is only an idea, which strikes more strongly upon me.  When I give the preference to one set of arguments above another, I do nothing but decide from my feeling concerning the superiority of their influence.  Objects have no discoverable connexion together; nor is it from any other principle but custom operating upon the imagination, that we can draw any inference from the appearance of one to the existence of another.