4. What, then, we have to ask, is the constant element in the first three entities? What is it that identifies them with their inherent Substance?
Is it the capacity to serve as a base? But Matter, we maintain, serves as the base and seat of Form: Form, thus, will be excluded from the category of Substance. Again, the Composite is the base and seat of attributes: hence, Form combined with Matter will be the basic ground of Composites, or at any rate of all posteriors of the Composite- Quantity, Quality, Motion, and the rest.
But perhaps we may think Substance validly defined as that which is not predicated of anything else. White and black are predicated of an object having one or other of these qualities; double presupposes something distinct from itself- we refer not to the half, but to the length of wood of which doubleness is affirmed. father qua father is a predicate; knowledge is predicated of the subject in whom the knowledge exists; space is the limit of something, time the measure of something. Fire, on the other hand, is predicated of nothing; wood as such is predicated of nothing; and so with man, Socrates, and the composite substance in general.
Equally the Substantial Form is never a predicate, since it never acts as a modification of anything. Form is not an attribute of Matter hence, is not predicable of Matter it is simply a constituent of the Couplement. On the other hand, the Form of a man is not different from the man himself [and so does not "modify" the Couplement].
Matter, similarly, is part of a whole, and belongs to something else only as to a whole and not as to a separate thing of which it is predicated. White, on the contrary, essentially belongs to something distinct from itself.
We conclude that nothing belonging to something else and predicated of it can be Substance. Substance is that which belongs essentially to itself, or, in so far as it is a part of the differentiated object, serves only to complete the Composite. Each or either part of the Composite belongs to itself, and is only affirmed of the Composite in a special sense: only qua part of the whole is it predicated of something else; qua individual it is never in its essential nature predicated of an external.
It may be claimed as a common element in Matter, Form and the Couplement that they are all substrates. But the mode in which Matter is the substrate of Form is different from that in which Form and the Couplement are substrates of their modifications.
And is it strictly true to say that Matter is the substrate of Form? Form is rather the completion which Matter's nature as pure potentiality demands.
Moreover, Form cannot be said to reside in Matter [as in a substrate]. When one thing combines with another to form a unity, the one does not reside in the other; both alike are substrates of a third: thus, Man [the Form] and a man [the Composite] are substrates of their experiences, and are prior to their activities and consequents.
Substance, then, is that from which all other things proceed and to which they owe their existence; it is the centre of passivity and the source of action.