Number then, whether regarded in itself or in the participant objects, belongs to the category of Quantity, but the participant objects do not. “Three yards long” does not fall under the category of Quantity, but only the three.
Why then are magnitudes classed as quantities? Not because they are so in the strict sense, but because they approximate to Quantity, and because objects in which magnitudes inhere are themselves designated as quantities. We call a thing great or small from its participation in a high number or a low. True, greatness and smallness are not claimed to be quantities, but relations: but it is by their apparent possession of quantity that they are thought of as relations. All this, however, needs more careful examination.
In sum, we hold that there is no single genus of Quantity. Only number is Quantity, the rest [magnitudes, space, time, motion] quantities only in a secondary degree. We have therefore not strictly one genus, but one category grouping the approximate with the primary and the secondary.
We have however to enquire in what sense the abstract numbers are substances. Can it be that they are also in a manner quantitative? Into whatever category they fall, the other numbers [those inherent in objects] can have nothing in common with them but the name.