It is in this wide sense that we use the term when we say that the (specific) nature (or “kind”) of a thing is revealed by its “qualities”; for the nature of a thing is revealed by all its accidents. And when we infer the nature of a thing from its activities, in accordance with the maxim _Qualis est operatio talis est natura_, we must take the term “_operatio_” or “activity” to include the operation of the thing on our cognitive faculties, the states of cognitive consciousness thus aroused in us, and all the other accidents thus revealed to us in the thing by its “knowledge-eliciting” action on our minds. But the term _Quality_ has been traditionally restricted, after Aristotle, to designate properly one particular category of accidents distinct from the others and from substance. A definition proper of any _genus supremum_ is of course out of the question. But it is not easy to give even a description which will convey an accurate notion of the special category of Quality, and mark it off from the other accident-categories. If we say with Aristotle that quality is “that whereby we are enabled to describe _what sort_ (ποιόν, _quale_) anything is”(323)—_e.g._ that it is white by whiteness, strong by strength, etc.—we are only illustrating the abstract by the concrete. But even this serves the purpose of helping us to realize what quality in general means. For we are more familiar with the concrete than with the abstract: and we can see a broad distinction between the question: “_What sort_ is that thing? _Qualis_ est ista res?” (Quality), and the question: “_How large_ is that thing? _Quanta_ est ista res?” (Quantity), or “_Where_ is that thing?” (Place), or “What is it _doing_? What is _happening_ to it?” (_Actio et Passio_), or “What does it _resemble_?” (_Relation_), etc. This will help us to realize that there are accidental modes of being which affect substances in a different way from all the extrinsic denominations of the latter (60), and also in a different way from Quantity, Relation, and Causality; and these modes of being, whereby the substance is _of such a sort_, or _in such a condition_, we call _qualities_. And if we inquire what special kind of _determination_ of the substance is common to qualities, and marks these off from the other accidents, we shall find it to consist in this, that quality is an accidental mode of being which so affects the substance that it disposes the latter well or ill in regard to the perfections natural to this particular kind of substance: it _alters_ the latter accidentally by increasing or diminishing its natural perfection. We have seen that no created substance has all the perfection natural to its kind, _tota simul_ or _ab initio_ (46); that it fulfils its rôle in existence by development, by tending towards its full or final perfection. The accidental realities which supervene on its essence, and thus _alter_ its perfection _within