I must not conclude this subject without observing, that 'tis 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_, may be 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 surprised 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 'tis 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, 'tis 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 may believe the existence of an impression and of an idea.