Book 10
7
There is a sense in which fire is an element (and no doubt so too is the indeterminateThe reference is undoubtedly to Anaximander. or some other similar thing, of its own nature), and there is a sense in which it is not; because to be fire and to be an element are not the same. It is as a concrete thing and as a stuff that fire is an element; but the term element denotes that it has this attribute: that something is made of it as a primary constituent.
8
The same is true of cause or one and all other such terms.
8
Hence to be one means to be indivisible (being essentially a particular thing, distinct and separate in place or form or thought), or to be whole and indivisible; but especially to be the first measure of each kind, and above all of quantity; for it is from this that it has been extended to the other categories.
9
Measure is that by which quantity is known, and quantity qua quantity is known either by unity or by number, and all number is known by unity. Therefore all quantity qua quantity is known by unity, and that by which quantities are primarily known is absolute unity.
10
Thus unity is the starting point of number qua number. Hence in other cases too measure means that by which each thing is primarily known, and the measure of each thing is a unit—in length, breadth, depth, weight and speed.
