When we gradually follow an object in its successive changes, the smooth progress of the thought makes us ascribe an identity to the succession; because it is by a similar act of the mind we consider an unchangeable object.

When we compare its situation after a considerable change the progress of the thought is broke; and consequently we are presented with the idea of diversity: In order to reconcile which contradictions the imagination is apt to feign something unknown and invisible, which it supposes to continue the same under all these variations; and this unintelligible something it calls a substance, or original and first matter.
We entertain a like notion with regard to the simplicity of substances, and from like causes.

Suppose an object perfectly simple and indivisible to be presented, along with another object, whose co-existent parts are connected together by a strong relation, it is evident the actions of the mind, in considering these two objects, are not very different.

The imagination conceives the simple object at once, with facility, by a single effort of thought, without change or variation.

The connexion of parts in the compound object has almost the same effect, and so unites the object within itself, that the fancy feels not the transition in passing from one part to another.