Qualitative change is wider than material change, for it includes changes in spiritual beings, _i.e._ in beings which are outside the category of quantity and have a mode of existence altogether different from the extensional, spatial existence which characterizes matter. When, for instance, the human mind acquires knowledge, it undergoes qualitative change. But matter, too, has qualities, and is subject to qualitative change. It is endowed with _active_ qualities, _i.e._ with powers, forces, energies, whereby it can not merely perform mechanical work by producing local changes in the distribution of its mass throughout space, but also produce physical and chemical changes which seem at least to be different in their nature from mere mechanical changes. It is likewise endowed with _passive_ qualities which appear to the senses to be of various kinds, differing from one another and from the mechanical or quantitative characteristics of size, shape, motion, rest, etc. While these latter are called “primary qualities” of bodies—because conceived to be more fundamental and more closely inherent in the real and objective nature of matter—or “common sensibles” (_sensibilia communia_), because perceptible by more than one of our external senses—the former are called “secondary qualities,” because conceived to be less characteristic of the real and objective nature of matter, and more largely subjective products of our own sentient cognitive activity—or “proper sensibles” (_sensibilia propria_), because each of them is apprehended by only one of our external senses: colour, sound, taste, odour, temperature, material state or texture (_e.g._ roughness, liquidity, softness, etc.). Now about all these perceived qualities and their changes the question has been raised: Are they, as such, _i.e._ as perceived by us, really in the material things or bodies which make up the physical universe, and really different in these bodies from the quantitative factors and motions of the latter? Or, as such, are they not rather partially or wholly subjective phenomena—products, at least in part, of our own sense perception, states of our own consciousness, having nothing really corresponding to them in the external matter of the universe beyond the quantitative, mechanical factors and motions whereby matter acts upon our faculties of sense cognition and produces these states of consciousness in us? This is a question of the first importance, the solution of which belongs to Epistemology. Aristotle would not allow that the objective material universe can be denuded, in the way just suggested, of qualities and qualitative change; and scholastic philosophers have always held the same general view. What we have to note here, however, in regard to the question is simply this, that even if the world of matter were thus simplified by transferring all qualitative change to the subjective domain of consciousness, the reality of qualitative change and all the problems