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.
After this any one will understand how we may form the idea of an impression and of an idea, and how we way believe the existence of an impression and of an idea.
SECT.IX.OF THE EFFECTS OF OTHER RELATIONS AND OTHER HABITS.
However convincing the foregoing arguments may appear, we must not rest contented with them, but must turn the subject on every side, in order to find some new points of view, from which we may illustrate and confirm such extraordinary, and such fundamental principles.

A scrupulous hesitation to receive any new hypothesis is so laudable a disposition in philosophers, and so necessary to the examination of truth, that it deserves to be complyed with, and requires that every argument be produced, which may tend to their satisfaction, and every objection removed, which may stop them in their reasoning.
I have often observed, that, beside cause and effect, the two relations of resemblance and contiguity, are to be considered as associating principles of thought, and as capable of conveying the imagination from one idea to another.