Hence the colour, taste, figure, solidity, and other qualities, combined in a peach or melon, are conceived to form one thing; and that on account of their close relation, which makes them affect the thought in the same manner, as if perfectly uncompounded.

But the mind rests not here.

Whenever it views the object in another light, it finds that all these qualities are different, and distinguishable, and separable from each other; which view of things being destructive of its primary and more natural notions, obliges the imagination to feign an unknown something, or original substance and matter, as a principle of union or cohesion among these qualities, and as what may give the compound object a title to be called one thing, notwithstanding its diversity and composition.
The peripatetic philosophy asserts the original matter to be perfectly homogeneous in all bodies, and considers fire, water, earth, and air, as of the very same substance; on account of their gradual revolutions and changes into each other.

At the same time it assigns to each of these species of objects a distinct substantial form, which it supposes to be the source of all those different qualities they possess, and to be a new foundation of simplicity and identity to each particular species.
All depends on our manner of viewing the objects.

When we look along the insensible changes of bodies, we suppose all of them to be of the same substance or essence.