remaining 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 join, but each is separate and distinct from the rest.

  A line, on the other hand, is a continuous quantity, for it is
possible to find a common boundary at which its parts join. In the
case of the line, this common boundary is the point; in the case of
the plane, it is the line: for the parts of the plane have also a
common boundary. Similarly you can find a common boundary in the
case of the parts of a solid, namely either a line or a plane.

  Space and time also belong to this class of quantities. Time,
past, present, and future, forms a continuous whole. Space,
likewise, is a continuous quantity; for the parts of a solid occupy
a certain space, and these have a common boundary; it follows that the
parts of space also, which are occupied by the parts of the solid,
have the same common boundary as the parts of the solid. Thus, not
only time, but space also, is a continuous quantity, for its parts
have a common boundary.

  Quantities consist either of parts which bear a relative position