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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.






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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