arising from it would still persist. To transfer qualitative change from object to subject, from matter to mind, is certainly something very different from explaining it as reducible to quantitative or mechanical change. The simplification thus effected would be more apparent than real: it would be simplifying the world of matter by transferring its complexity to the world of mind. This consideration is one which is sometimes lost sight of by scientists who advance mechanical hypotheses as ultimate explanations of the nature and activities of the physical universe. If all material things and processes could be ultimately analysed into configurations and local motions of space-occupying atoms, homogeneous in nature and differing only in size and shape, then each of these ultimate atomic factors would be itself exempt from intrinsic change as to its own essence and individuality. In this hypothesis there would be really no such thing as _substantial_ change. The collection of atoms would form an immutable core of material reality, wholly simple and ever actual. Such an hypothesis, however, is utterly inadequate as an explanation of the facts of life and consciousness. And even as an account of the processes of the inorganic universe it encounters insuperable difficulties. The common belief of men has always been that even in this domain of reality there are fundamentally different _kinds_ of matter, kinds which differ from one another not merely in the shape and size and configuration and arrangement of their ultimate _actual_ constituents, but even in the very substance or nature of these constituents; and that there are some material changes which affect the actual substance itself of the matter which undergoes them. This belief scholastics, again following Aristotle, hold to be a correct belief, and one which is well grounded in reason. And this belief in turn involves the view that every type of actual material entity—whether merely inorganic, or endowed with life, or even allied with a higher, spiritual mode of being as in the case of man himself—is _essentially composite_, essentially a synthesis of _potential_ and _actual_ principles of being, and therefore capable of _substantial_ change. The actually existing material being scholastics describe as _materia secunda_, the ὕλη ἐσχάτη of Aristotle; the purely potential factor, which is actualized in this or that particular kind of matter, they describe as _materia prima_, the ὕλη πρώτη of Aristotle; the actualizing, specifying, formative principle, they designate as _forma substantialis_ (εἶδος). And since the purely potential principle cannot actually exist except as actualized by some formative principle, all substantial change or transition from one substantial type to another is necessarily both a _corruptio_ and a _generatio_. That is, it involves the actual disappearance of one substantial form and the actual appearance of another.