For when we consider any two points of this time, we may place them in different lights: We may either survey them at the very same instant; in which case they give us the idea of number, both by themselves and by the object; which must be multiplyd, in order to be conceivd at once, as existent in these two different points of time: Or on the other hand, we may trace the succession of time by a like succession of ideas, and conceiving first one moment, along with the object then existent, imagine afterwards a change in the time without any VARIATION or INTERRUPTION in the object; in which case it gives us the idea of unity.
Here then is an idea, which is a medium betwixt unity and number; or more properly speaking, is either of them, according to the view, in which we take it: And this idea we call that of identity.

We cannot, in any propriety of speech, say, that an object is the same with itself, unless we mean, that the object existent at one time is the same with itself existent at another.

By this means we make a difference, betwixt the idea meant by the word, OBJECT, and that meant by ITSELF, without going the length of number, and at the same time without restraining ourselves to a strict and absolute unity.
Thus the principle of individuation is nothing but the INVARIABLENESS and UNINTERRUPTEDNESS of any object, thro a supposd variation of time, by which the mind can trace it in the different periods of its existence, without any break of the view, and without being obligd to form the idea of multiplicity or number.
I now proceed to explain the SECOND part of my system, and shew why the constancy of our perceptions makes us ascribe to them a perfect numerical identity, tho there be very long intervals betwixt their appearance, and they have only one of the essential qualities of identity, VIZ, INVARIABLENESS.

That I may avoid all ambiguity and confusion on this head, I shall observe, that I here account for the opinions and belief of the vulgar with regard to the existence of body; and therefore must entirely conform myself to their manner of thinking and of expressing themselves.

Now we have already observd, that however philosophers may distinguish betwixt the objects and perceptions of the senses; which they suppose co-existent and resembling; yet this is a distinction, which is not comprehended by the generality of mankind, who as they perceive only one being, can never assent to the opinion of a double existence and representation.