Book 5
12
Other things are said to be potentsc. in a passive sense, which the English word potent cannot bear. because something else has such a potency over them; others because it does not possess it; others because it possesses it in a particular way. The term impotent is similarly used. Thus the authoritative definition of potency in the primary sense will be a principle producing change, which is in something other than that in which the change takes place, or in the same thing qua other.
1
Quantity means that which is divisible into constituent parts, eachi.e., if there are only two. or every one of which is by nature some one individual thing. Thus plurality, if it is numerically calculable, is a kind of quantity; and so is magnitude, if it is measurable. Plurality means that which is potentially divisible into non-continuous parts; and magnitude that which is potentially divisible into continuous parts. Of kinds of magnitude, that which is continuous in one direction is length; in two directions, breadth; in three, depth.
2
And of these, plurality, when limited, is a number; length, a line; breadth, a plane; depth, a body. Again, some things are essentially quantitative, but others only accidentally; e.g. the line is essentially, but cultured accidentally quantitative.
3
And of the former class some are quantitative in virtue of their substance, e.g. the fine (because the definition which describes it is quantitative in some form); and others are attributes and conditions of a substance of this kind— e.g., much and little, long and short, broad and narrow, deep and shallow, heavy and light, etc.
4
Moreover, great and small, and greater and smaller, whether used absolutely or relatively to one another, are essential attributes of quantity; by an extension of meaning, however, these terms are also applied to other things.
