self 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.

Part 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