This is so absolutely necessary, that without it there could be no knowledge, no reasoning, no imagination, no distinct thoughts at all.

By this the mind clearly and infallibly perceives each idea to agree with itself, and to be what it is; and all distinct ideas to disagree, i.e.the one not to be the other: and this it does without pains, labour, or deduction; but at first view, by its natural power of perception and distinction.

And though men of art have reduced this into those general rules, WHAT IS, IS, and IT IS IMPOSSIBLE FOR THE SAME THING TO BE AND NOT TO BE, for ready application in all cases, wherein there may be occasion to reflect on it: yet it is certain that the first exercise of this faculty is about particular ideas.

A man infallibly knows, as soon as ever he has them in his mind, that the ideas he calls WHITE and ROUND are the very ideas they are; and that they are not other ideas which he calls RED or SQUARE.

Nor can any maxim or proposition in the world make him know it clearer or surer than he did before, and without any such general rule.