28. Neither can we say how far the identity of the corporeal substance continues under its different transformations. The partisans of corpuscular philosophy consider all transformations as mere local motions, and all the variations which we see in bodies as mere results of the different position of the corpuscles among themselves. Leibnitz resolved matter into an infinity of monads, differing from the atoms of Epicurus, but conducing to the substantial invariability of bodies, which are only a collection of indivisible substances, which he calls monads. The Aristotelians believed that, of the changes of bodies, some were accidental, as figure, motion, density, warmth, cold, etc.; others substantial, as the change of wood to ashes. But in all the variety of systems, all admit something permanent, the subject of the changes. The Atomists and Leibnitz evidently admitted the identity of the subject. As to the Aristotelians, although the change which introduced a substantial form different from the first substantially transformed the being, so that after the change of the substantial form it could not be said that one was substantially the other, they still thought there was a common subject in these substantial transformations, and this was what they called the first matter, _materia prima_. All systems of philosophy admit this clear and evident truth, that in the midst of the transformations of the corporeal world, there is something permanent. 29. This corporeal substance being a reality, must not only exist, but it must be something determinate. This substantial determination of the body, which makes it this particular thing, and distinguishes it in its internal nature, in its essence, from all other bodies of other species, the Aristotelians called the substantial form. The subject of this form, or actuality, which was common to all bodies, they called the _materia prima_, which was a pure potentiality, a sort of medium between pure nothing and actual being.