Proceeding next to ask in what form the problem of philosophy presented itself to the Stoics, three points deserve to be specially noticed. 1. In the first place, philosophy was determined practically by an end in view. 2. The character of this end was decided by the idea of conformity with reason; and 3, this view was substantiated by intellectual proof. [(1) Its practical character.] The real business of all philosophy, according to the Stoics, is the moral conduct of man. Philosophy is the exercise of an art, and more particularly of the highest art—virtue: [68] it is therefore the learning of virtue. Now virtue can only be learnt by exercise, and therefore philosophy is at the same time virtue, [69] and the several parts of philosophy are so many distinct virtues. [70] Morality is the central point towards which all other enquiries converge. Even natural science, although lauded as the inmost shrine of philosophy, is, according to Chrysippus, only necessary for the philosopher to enable him to distinguish between things good and evil, between what should be done and what should be left undone. [71] So far from approving pure speculation, which Plato and Aristotle had commended as the height of human happiness, Chrysippus plainly asserted that to live for speculation is equivalent to living only for pleasure. [72] With this view of Chrysippus most of the statements of the Stoics as to the relation of various branches of philosophy to each other agree, although there is a certain amount of vagueness about them, owing to reasons which will shortly be mentioned; and on no other hypothesis can the internal structure and foundation of their system be satisfactorily explained. It is enough to remark here, as has been done before, [73] that the most important and most distinctive points established by the Stoic School belong to the sphere of ethics. In logic and natural science the School displays far less independence, for the most part following older teachers; and it is expressly noted, as a deviation from the ordinary teaching of the School, that Herillus, the pupil of Zeno, declared knowledge to be the highest good, thus making it the chief end in philosophy. [74] [(2) Necessity for intellectual knowledge.]