The notion of quality applies to spiritual as well as material things. Since, however, the work of defining and assigning this notion falls upon metaphysics, we shall be brief in that regard and attend more to the question of its reality or objectivity.
Quality, in general, is easier to experience than to define. As a matter of fact, the experience is so primary and universal that it can scarcely be reduced to anything simpler. Yet quality has been given a manner of definition. "By 'quality,' " says Aristotle, "I mean that in virtue of which things are said to be such and such." 6 This is a very broad rendition of quality, broad enough to include substantial differences, by which things are essentially dissimilar. In its more usual, and proper, meaning quality denotes an accidental modification or specification of what is already substantially or essentially complete. This is the sense we now speak of it.
On this subject of quality there is apparently complete opposition between Aristotle's physics and those scienceimbued systems of thought that are customarily lumped together under the label mechanism, a loose term at best. In any event the mechanists, for want of a better name, distinguish qualities into primary and secondary. The former include such aspects as extension, shape, and motion; the latter, such properties as color, taste, and smell. So far so good, except that in Aristotelian psychology the primary qualities of the mechanists are in fact secondary, and conversely.
More important is the mechanists' allegation that only primary qualities are objectively real, and their consequent disregard of secondary qualities in the explanation of the physical world. Thus, all interpretation of nature becomes mathematical - or so the mechanists persuade themselves. It bears mentioning, though, that even the extremest mechanism never achieved the complete suppression of nature's qualitative features. The atoms of Democritus still had shape; and the amorphous extension of Cartesian physics was not a universe or cosmos except by differentiat- ing motions. Therefore mechanism, despite its claim, does not spell the total liquidation of nature's qualitative aspects; rather, it underscores the age-old desire to conquer this more elusive order of things by sifting and reducing it to its simplest features.
Aristotle, it is clear, could not accept the mechanistic account of quality without considerable pruning or adjustment. In his view, qualities, even as perceived, are objectively real. Not only that, but quality is the immediate principle of all physical change in nature, since "alteration," which is movement according to quality, provides the immediate disposition to such change. In Aristotle's study of nature quality, accordingly, assumes a role and importance which it does not have in the mechanistic tradition.
Much could be said about this diversity of outlook; much, also, that would be beside the point. One thing to be borne in mind is that while Aristotle and the mechanists - perhaps we should say "scientists" - seek the same tophies, the secrets of nature, their search is from different levels. The scientists may prefer to study nature from the quantitative aspect, which lends itself to more exact measurement. This course is perfectly legitimate, though in his preoccupation with the quantitative side of things an investigator, wittingly or not, may minimize and oversimply the qualitative features. But the quantitative explanation of nature is never the whole story; it is a selective report. This, too, needs to be remembered.
Above all, it must not be supposed that the quantitative discovery of nature is a philosophy of nature, an expression of the total reality down to its ultimate principles. In the philosophical study of nature quality will hardly be less important than quantity. As a matter of fact, even in science proper (in the modern sense) the recognition is growing that quality cannot be utterly neglected. The time is over when mechanism (with its imagined elimination of quality) could commend itself to scientists as an exhaustive explanation of the physical world. The significance of this change in attitude should be apparent. Today, a philosophy of nature in which (as in Aristotle's) quality has a primordial role cannot be counted out on the plea of inassociability with scientific outlook.