The first is our present subject; and to explain it perfectly we must take the matter pretty deep, and account for that identity, which we attribute to plants and animals; there being a great analogy betwixt it, and the identity of a self or person.
We have a distinct idea of an object, that remains invariable and uninterrupted through a supposed variation of time; and this idea we call that of identity or sameness.

We have also a distinct idea of several different objects existing in succession, and connected together by a close relation; and this to an accurate view affords as perfect a notion of diversity, as if there was no manner of relation among the objects.

But though these two ideas of identity, and a succession of related objects be in themselves perfectly distinct, and even contrary, yet it is certain, that in our common way of thinking they are generally confounded with each other.

That action of the imagination, by which we consider the uninterrupted and invariable object, and that by which we reflect on the succession of related objects, are almost the same to the feeling, nor is there much more effort of thought required in the latter case than in the former.

The relation facilitates the transition of the mind from one object to another, and renders its passage as smooth as if it contemplated one continued object.