The inconsistencies of a great philosophical system are best explained by examining its historical antecedents. We have already attempted to disentangle the roots from which Stoicism was nourished, but one of the most important has not yet been taken into account. This was the still continued influence of Parmenides, derived, if not from his original teaching, then from some one or more of the altered shapes through which it had passed. It has been shown how Zeno used the Heracleitean method to break down all the demarcations laboriously built up by Plato and Aristotle. Spirit was identified with matter; ideas with aerial currents; God with the world; rational with sensible evidence; volition with judgment; and emotion with thought. But the idea of a fundamental antithesis, expelled from every other department of enquiry, took hold with all the more energy on what, to Stoicism, was the most vital of all distinctions—that between right and wrong.[57] Once grasp this transformation of a metaphysical into a moral principle, and every paradox of the system will be seen to follow from it with logical necessity. What the supreme Idea had been to Plato and self-thinking thought to Aristotle, that virtue became to the new school, simple, unchangeable, and self-sufficient. It must not only be independent of pleasure and pain, but absolutely incommensurable with them; therefore there can be no happiness except what it gives. As an indivisible unity, it must be possessed entirely or not at all; and being eternal, once possessed it can never be lost. Further, since the same action may be either right or wrong, according to the motive of its performance, virtue is nothing external, but a subjective disposition, a state of the will and the affections; or, if these are to be considered as judgments, a state of the reason. Finally, since the universe is organised reason, virtue must be natural, and especially consonant to the nature of man as a rational animal; while, at the same time, its existence in absolute purity being inconsistent with experience, it must remain an unattainable ideal. It has been shown in former parts of this work how Greek philosophy, after straining an antithesis to the utmost, was driven by the very law of its being to close or bridge over the chasm by a series of accommodations and transitions. To this rule Stoicism was no exception; and perhaps its extraordinary vitality may have been partly due to the necessity imposed on its professors of continually revising their ethics, with a view to softening down its most repellent features. We proceed to sketch in rapid outline the chief artifices employed for this purpose.