others of the sort), in virtue of which, when they change, bodies are said to alter. (4) Quality in respect of virtue and vice and, in general, of evil and good.
Quality, then, seems to have practically two meanings, and one of these is the more proper. The primary quality is the essential differentia, and of this the quality in numbers is a part; for it is a differentia of essences, but either not of things in motion or not of them qua in motion. Secondly, there are the modifications of things in motion qua in motion, and the differentiae of movements. Virtue and vice fall among these modifications; for they indicate differentiae of the movement or activity, according to which the things in motion act or are acted on well or badly; for that which can be moved or act in one way is good, and that which can do so in another — the contrary — way is vicious. Good and evil indicate quality especially in living things, and among these especially in those which have purpose.
Chapter 15
Things are relative (1) as double to half and treble to a third, and in general that which contains something else many times to that which is contained many times in something else, and that which exceeds to that which is exceeded; (2) as that which can heat to that which can be heated, and that which can cut to that which can be cut, and in general the active to the passive; (3) as the measured to the measure and the known to knowledge and the perceived to perception.
(1) Relative terms of the first kind are numerically related either indefinitely or definitely, either to various numbers or to 1, e.g. the double is in a definite numerical relation to 1, and that which is 'many times as great' is in a numerical, but not in a definite, relation to 1, i.e. not in this or in that relation to it; the relation of that which is of something else to that something is a definite numerical relation to a number; that which is n + 1 / n times something else is in an indefinite relation to that some-
