THE PHYSICAL SYSTEM OF ST. THOMAS.

I. THE ESSENCE AND NATURE OF CORPOREAL SUBSTANCES.

THERE is nothing perhaps harder to defend and easier to attack than a system not clearly defined, at least in its principal parts. Its defenders waste their time in showing what is either beside the question or only touches the surface, and its adversaries, when they have pointed out the weak points in that which, true or false, has nothing to do with the truth of the system, settle the controversy in their own favour.

To avoid this, we shall explain without delay the system of which we are going to treat; and, first of all, we must remark that it differs from the Mechanic and Dynamic systems as to the very essence of corporeal substances. According to the Mechanic system, corporeal substance means inert and resisting atoms aggregated in varying order. The Dynamic system recognizes subsisting forces only, as in mathematical points of space. The Physical system supposes that every individual corporeal substance is essentially composed of two principles really distinct. One is the source of extension and is called materia prima. The other is the source of activity, and is called the substantial form. On the variety of the latter the diversities of nature in every substance depend. The materia prima and the substantial form, being incomplete substances, cannot be apart. God did not create matter quite without form, but endowed it with diverse forms actuated with virtues that may be called seminal, radically containing the whole order and beauty of the sensible universe, which by degrees developed and in the course of time is continually developing. During this continual development we perceive changes in substances, in their qualities and in their accidents, and in their mutual approach and departure. In the former case there is a true change of substantial forms, the matter of one body being transmuted into that of another. In the second there is a change of accidental forms only. In the third, unless there is some mechanical impulse, one body is brought towards another by true attraction. From such forms and such properties, impressed on creation, a determinate development of the corporeal universe necessarily follows; and in these forms and in these properties we must recognize the existence of physical laws, considered, not as in the Mind or Will of the Creator, but as an effect in the created things themselves.

But this is too rapid a sketch. We must consider each part of the system in detail.