Book excerpt: The Stoics taught that destructive emotions resulted from errors in judgment, of the active relationship between cosmic determinism and human freedom, and the belief that it is virtuous to maintain a will that is in accord with nature. Because of this, the Stoics presented their philosophy as a way of life, and they thought that the best indication of an individual's philosophy was not what a person said but how that person behaved. To live a good life, one had to understand the rules of the natural order since they taught that everything was rooted in nature. Later Stoics-such as Seneca and Epictetus-emphasized that, because "virtue is sufficient for happiness," a sage was immune to misfortune. This belief is similar to the meaning of the phrase "stoic calm," though the phrase does not include the "radical ethical" Stoic views that only a sage can be considered truly free, and that all moral corruptions are equally vicious. From its founding, Stoic doctrine was popular with a following in Roman Greece and throughout the Roman Empire-including the Emperor Marcus Aurelius.