Chapter 9

Substance-principle, substance-term and mixed substance

842. Hence, the first act perceived in the object of perception is substance. But this first act is sometimes understood as principle, sometimes as term. Moreover, this first act ('first' must always be understood relative to the intrinsic order of the perceived or conceived entity) sometimes presents itself as essentially and solely principle; sometimes as essentially and solely term of the very act whose principle remains hidden from us; sometimes as containing in itself the twofold relationship of term of one act and principle of another.

843. There are, therefore, three species of substance. One is first act (in the conceived object) which has and never loses the nature of principle. Another is first act (in the conceived object) which has and never loses the nature of term. The third is first act (in the conceived object) which is understood as term relative to an act preceding it (a different substance, therefore, from itself), and as principle relative to its own and subsequent acts of which alone it is first act and act-principle.

844. To be essentially and solely act-principle pertains to God alone; to be essentially and solely act-term pertains to material substance; to be first act but term relative to some preceding activity, and principle relative to the act of one's own subsistence and to second acts, pertains to spiritual creatures, and therefore to the human soul.

845. We need to note carefully that this is the classification of substances, or first acts (where we understand 'first' in the sense of what we conceive of an ens, according to the logical order, prior to other things in it). It is not a classification of acts in general. If we forget this, we may object that bodies, too, are principles of their operations. This is not the case. In all extrasubjective operations of bodies, it is always the term that is under consideration. Hence our previous proof of the inertia of matter. Changes in bodies are not, therefore, operations of corporeal substance, but modifications of it. This activity is always perceived in its term, never in its principle.