"Quality" means (1) the differentia of the essence, e.g. man is an
animal of a certain quality because he is two-footed, and the horse is
so because it is four-footed; and a circle is a figure of particular
quality because it is without angles,-which shows that the essential
differentia is a quality.-This, then, is one meaning of quality-the
differentia of the essence, but (2) there is another sense in which it
applies to the unmovable objects of mathematics, the sense in which
the numbers have a certain quality, e.g. the composite numbers which
are not in one dimension only, but of which the plane and the solid
are copies (these are those which have two or three factors); and in
general that which exists in the essence of numbers besides quantity
is quality; for the essence of each is what it is once, e.g. that of
is not what it is twice or thrice, but what it is once; for 6 is
once 6.

    (3) All the modifications of substances that move (e.g. heat and
cold, whiteness and blackness, heaviness and lightness, and the 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 differentia of
the essence, and of this the quality in numbers is a part; for it is a
differentia of essences, but either not of things that move or not
of them qua moving. Secondly, there are the modifications of things
that move, qua moving, 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.