cannot in this case be esteemed the effect of custom.  But this difficulty will vanish, if we consider, that though we are here supposed to have had only one experiment of a particular effect, yet we have many millions to convince us of this principle; that like objects placed in like circumstances, will always produce like effects; and as this principle has established itself by a sufficient custom, it bestows an evidence and firmness on any opinion, to which it can be applied.  The connexion of the ideas is not habitual after one experiment:  but this connexion is comprehended under another principle, that is habitual; which brings us back to our hypothesis.  In all cases we transfer our experience to instances, of which we have no experience, either expressly or tacitly, either directly or indirectly.

I must not conclude this subject without observing, that it is very difficult to talk of the operations of the mind with perfect propriety and exactness; because common language has seldom made any very nice distinctions among them, but has generally called by the same term all such as nearly resemble each other.  And as this is a source almost inevitable of obscurity and confusion in the author; so it may frequently give rise to doubts and objections in the reader, which otherwise he would never have dreamed of.  Thus my general position, that an opinion or belief is nothing but a strong and lively idea derived from a present impression related to it, maybe liable to the following objection, by reason of a little ambiguity in those words strong and lively.  It may be said, that not only an impression may give rise to reasoning, but that an idea may also have the same influence; especially upon my principle, that all our ideas are derived from correspondent impressions.  For suppose I form at present an idea, of which I have forgot the correspondent impression, I am able to conclude from this idea, that such an impression did once exist; and as this conclusion is attended with belief, it may be asked, from whence are the qualities of force and vivacity derived, which constitute this belief?  And to this I answer very readily, from the present idea.  For as this idea is not here considered, as the representation of any absent object, but as a real perception in the mind, of which we are intimately conscious, it must be able to bestow on whatever is related to it the same quality, call it firmness, or solidity, or force, or vivacity, with which the mind reflects upon it, and is assured of its present existence.  The idea here supplies the place of an impression, and is entirely the same, so far as regards our present purpose.

Upon the same principles we need not be surprized to hear of the remembrance of an idea:  that is, of the idea of an idea, and of its force and vivacity superior to the loose conceptions of the imagination.  In thinking of our past thoughts we not only delineate out the objects, of which we were thinking, but also conceive the action of the mind in the meditation, that certain JE-NE-SCAI-QUOI, of which it is impossible to give any definition or description, but which every one sufficiently understands.  When the memory offers an idea of this, and represents it as past, it is easily conceived how that idea may have more vigour and firmness, than when we think of a past thought, of which we have no remembrance.