[Footnote 5.  Here are the heads of those arguments, which lead us to this conclusion.  When we infer the existence of an object from that of others, some object must always be present either to the memory or senses, in order to be the foundation of our reasoning; since the mind cannot run up with its inferences in infinitum.  Reason can never satisfy us that the existence of any one object does ever imply that of another; so that when we pass from the impression of one to the idea or belief of another, we are not determined by reason, but by custom or a principle of association.  But belief is somewhat more than a simple idea.  It is a particular manner of forming an idea:  And as the same idea can only be varyed by a variation of its degrees of force and vivacity; it follows upon the whole, that belief is a lively idea produced by a relation to a present impression, according to the foregoing definition.]

This operation of the mind, which forms the belief of any matter of fact, seems hitherto to have been one of the greatest mysteries of philosophy; though no one has so much as suspected, that there was any difficulty in explaining it.  For my part I must own, that I find a considerable difficulty in the case; and that even when I think I understand the subject perfectly, I am at a loss for terms to express my meaning.  I conclude, by an induction which seems to me very evident, that an opinion or belief is nothing but an idea, that is different from a fiction, not in the nature or the order of its parts, but in the manner of its being conceived.  But when I would explain this manner, I scarce find any word that fully answers the case, but am obliged to have recourse to every one’s feeling, in order to give him a perfect notion of this operation of the mind.  An idea assented to feels different from a fictitious idea, that the fancy alone presents to us:  And this different feeling I endeavour to explain by calling it a superior force, or vivacity, or solidity, or firmness, or steadiness.  This variety of terms, which may seem so unphilosophical, is intended only to express that act of the mind, which renders realities more present to us than fictions, causes them to weigh more in the thought, and gives them a superior influence on the passions and imagination.  Provided we agree about the thing, it is needless to dispute about the terms.  The imagination has the command over all its ideas, and can join, and mix, and vary them in all the ways possible.  It may conceive objects with all the circumstances of place and time.  It may set them, in a, manner, before our eyes in their true colours, just as they might have existed.  But as it is impossible, that that faculty can ever, of itself, reach belief, it is evident, that belief consists not in the nature and order of our ideas, but in the manner of their conception, and in their feeling to the mind.  T confess, that it is impossible to explain perfectly