The importance attaching to this division of virtues, the ultimate basis on which it rests, and the relation which virtues bear, both to one another and to the common essence of virtue, are topics upon which Zeno never entered. Plutarch, at least, blames him [645] for treating virtues as many, and yet inseparable, and at the same time for finding in all virtues only certain manifestations of the understanding. Aristo attempted to settle this point more precisely. According to his view, virtue is in itself only one; in speaking of many virtues, we only refer to the variety of objects with which that one virtue has to do. [646] The difference of one virtue from another is not one of inward quality, but depends on the external conditions under which they are manifested; it only expresses a definite relation to something else, or, in the language of Herbart, an accidental aspect. [647] The same view would seem to be implied in the manner in which Cleanthes determines the relations of the principal virtues to one another. [648] It was, however, opposed by Chrysippus. The assumption of many virtues, he believed, rested upon an inward difference; [649] each definite virtue, as also each definite fault, becoming what it does by a peculiar change in the character of the soul itself; [650] in short, for a particular virtue to come into being, it is not enough that the constituent element of all virtue should be directed towards a particular object, but to the common element must be superadded a further characteristic element, or differentia; the several virtues being related to one another, as the various species of one genus.