This, then, is one meaning of quality differentia of substance, but there is another sense in which it applies to the unmovable objects of mathematics; ie the numbers have a certain quality, eg the composite numbers which are not in one dimension only, but of which the plane and the solid are copies (these are those which have two or three factors); and in general that which exists in the substance of numbers besides quantity is quality; for the substance of each is what it is once, eg that of is not what it is twice or thrice, but