The Complete Aristotle (eng.)

numerically one and the same, it is capable of admitting
contrary qualities. From among things other than substance, we
should find ourselves unable to bring forward any which possessed
this mark. Thus, one and the same colour cannot be white and black.
Nor can the same one action be good and bad: this law holds good
with everything that is not substance. But one and the selfsame
substance, while retaining its identity, is yet capable of
admitting contrary qualities. The same individual person is at one
time white, at another black, at one time warm, at another cold, at
one time good, at another bad. This capacity is found nowhere else,
though it might be maintained that a statement or opinion was an
exception to the rule. The same statement, it is agreed, can be
both true and false. For if the statement ‘he is sitting’ is true,
yet, when the person in question has risen, the same statement will
be false. The same applies to opinions. For if any one thinks truly
that a person is sitting, yet, when that person has risen, this
same opinion, if still held, will be false. Yet although this
exception may be allowed, there is, nevertheless, a difference in
the manner in which the thing takes place. It is by themselves
changing that substances admit contrary qualities. It is thus that
that which was hot becomes cold, for it has entered into a
different state. Similarly that which was white becomes black, and
that which was bad good, by a process of change; and in the same
way in all other cases it is by changing that substances are
capable of admitting contrary qualities. But statements and
opinions themselves remain unaltered in all respects: it is by the
alteration in the facts of the case that the contrary quality comes
to be theirs. The statement ‘he is sitting’ remains unaltered, but
it is at one time true, at another false, according to
circumstances. What has been said of statements applies also to
opinions. Thus, in respect of the manner in which the thing takes
place, it is the peculiar mark of substance that it should be
capable of admitting contrary qualities; for it is by itself
changing that it does so.
    If, then, a man should make this exception and contend that
statements and opinions are capable of admitting contrary
qualities, his contention is unsound. For statements and opinions
are said to have this capacity, not because they themselves undergo
modification, but because this modification occurs in the case of
something else. The truth or falsity of a statement depends on
facts, and not on any power on the part of the statement itself of
admitting contrary qualities. In short, there is nothing which can
alter the nature of statements and opinions. As, then, no change
takes place in themselves, these cannot be said to be capable of
admitting contrary qualities.
    But it is by reason of the modification which takes place within
the substance itself that a substance is said to be capable of
admitting contrary qualities; for a substance admits within itself
either disease or health, whiteness or blackness. It is in this
sense that it is said to be capable of admitting contrary
qualities.
    To sum up, it is a distinctive mark of substance, that, while
remaining numerically one and the same, it is capable of admitting
contrary qualities, the modification taking place through a change
in the substance itself.
    Let these remarks suffice on the subject of substance.
6
    Quantity is either discrete or continuous. Moreover, some
quantities are such that each part of the whole has a relative
position to the other parts: others have within them no such
relation of part to part.
    Instances of discrete quantities are number and speech; of
continuous, lines, surfaces, solids, and, besides these, time and
place.
    In the case of the parts of a number, there is no common
boundary at which they join. For example: two fives make ten, but
the two fives have no common boundary, but are separate; the parts
three and seven also do not join at any boundary. Nor, to
generalize, would it ever be possible in the case of number that
there should be a common boundary among the parts; they are always
separate. Number, therefore, is a discrete quantity.
    The same is true of speech. That speech is a quantity is
evident: for it is measured in long and short syllables. I mean
here that speech which is vocal. Moreover, it is a discrete
quantity for its parts have no common boundary. There is no common
boundary at which the syllables