6. We are led thus to the question of receptivity in things of body.
An additional proof that bodies must have some substratum different from themselves is found in the changing of the basic-constituents into one another. Notice that the destruction of the elements passing over is not complete – if it were we would have a Principle of Being wrecked in Non-being – nor does an engendered thing pass from utter non-being into Being: what happens is that a new form takes the place of an old. There is, then, a stable element, that which puts off one form to receive the form of the incoming entity.
The same fact is clearly established by decay, a process implying a compound object; where there is decay there is a distinction between Matter and Form.
And the reasoning which shows the destructible to be a compound is borne out by practical examples of reduction: a drinking vessel is reduced to its gold, the gold to liquid; analogy forces us to believe that the liquid too is reducible.
The basic-constituents of things must be either their Form-Idea or that Primal Matter (of the Intelligible) or a compound of the Form and Matter.
Form-Idea, pure and simple, they cannot be: for without Matter how could things stand in their mass and magnitude?
Neither can they be that Primal Matter, for they are not indestructible.
They must, therefore, consist of Matter and Form-Idea – Form for quality and shape, Matter for the base, indeterminate as being other than Idea.