However closely the Stoics cling to the idea that all virtue is based on knowledge, and is in itself nothing else but knowledge, they are not content with knowledge, or with placing knowledge above practical activity, as Plato and Aristotle had done. As we have seen already, knowledge with them was only a means towards rational conduct, [624] and it is expressly mentioned, as a deviation from the teaching of the School, that Herillus of Carthage, Zeno’s pupil, declared knowledge to be the end of life, and the only unconditional good. [625] Virtue may, it is true, be called knowledge, but it is, at the same time, essentially health and strength of mind, a right state of the soul agreeing with its proper nature; [626] and it is required of man that he should never cease to labour and contribute towards the common good. [627] Thus, according to Stoic principles, virtue is a combination of theory and practice, in which action is invariably based on intellectual knowledge, but, at the same time, knowledge finds its object in moral conduct—it is, in short, power of will based on rational understanding. [628] This definition must not, however, be taken to imply that moral knowledge precedes will, and is only subsequently referred to will, nor conversely that the will only uses knowledge as a subsidiary instrument. In the eyes of a Stoic, knowledge and will are not only inseparable, but they are one and the same thing. Virtue cannot be conceived without knowledge, nor knowledge without virtue. [629] The one, quite as much as the other, is a right quality of the soul, or, speaking more correctly, is the rightly endowed soul,—reason, when it is as it ought to be. [630] Hence virtue may be described, with equal propriety, either as knowledge or as strength of mind; and it is irrelevant to enquire which of these two elements is anterior in point of time. [(b) The virtues severally.]