The Complete Aristotle (eng.)

change of meaning.-These
senses of ‘capable’ or ‘possible’ involve no reference to potency.
But the senses which involve a reference to potency all refer to
the primary kind of potency; and this is a source of change in
another thing or in the same thing qua other. For other things are
called ‘capable’, some because something else has such a potency
over them, some because it has not, some because it has it in a
particular way. The same is true of the things that are incapable.
Therefore the proper definition of the primary kind of potency will
be ‘a source of change in another thing or in the same thing qua
other’.
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13
    ‘Quantum’ means that which is divisible into two or more
constituent parts of which each is by nature a ‘one’ and a ‘this’.
A quantum is a plurality if it is numerable, a magnitude if it is a
measurable. ‘Plurality’ means that which is divisible potentially
into non-continuous parts, ‘magnitude’ that which is divisible into
continuous parts; of magnitude, that which is continuous in one
dimension is length; in two breadth, in three depth. Of these,
limited plurality is number, limited length is a line, breadth a
surface, depth a solid.
    Again, some things are called quanta in virtue of their own
nature, others incidentally; e.g. the line is a quantum by its own
nature, the musical is one incidentally. Of the things that are
quanta by their own nature some are so as substances, e.g. the line
is a quantum (for ‘a certain kind of quantum’ is present in the
definition which states what it is), and others are modifications
and states of this kind of substance, e.g. much and little, long
and short, broad and narrow, deep and shallow, heavy and light, and
all other such attributes. And also great and small, and greater
and smaller, both in themselves and when taken relatively to each
other, are by their own nature attributes of what is quantitative;
but these names are transferred to other things also. Of things
that are quanta incidentally, some are so called in the sense in
which it was said that the musical and the white were quanta, viz.
because that to which musicalness and whiteness belong is a
quantum, and some are quanta in the way in which movement and time
are so; for these also are called quanta of a sort and continuous
because the things of which these are attributes are divisible. I
mean not that which is moved, but the space through which it is
moved; for because that is a quantum movement also is a quantum,
and because this is a quantum time is one.
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14
    ‘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