The Stoics are especially known for teaching that "virtue is the only good" for human beings, and that external things—such as health, wealth, and pleasure—are not good or bad in themselves, but have value as "material for virtue to act upon". Alongside Aristotelian ethics, the Stoic tradition forms one of the major founding approaches to Western virtue ethics. The Stoics also held that certain destructive emotions resulted from errors of judgment, and they believed people should aim to maintain a will (called prohairesis) that is "in accord with nature" (a slogan they interpreted to mean several different things). They thought the best indication of an individual's philosophy was not what a person said, but how a person behaved. To live a good life, one had to understand the rules of the natural order since they taught everything was rooted in nature.
Many Stoics—such as Seneca and Epictetus—emphasized that because "virtue is sufficient for happiness", a sage would be emotionally resilient 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 during the Roman Empire—and its adherents included the Emperor Marcus Aurelius. It later experienced a decline after Christianity had become the state religion in the 4th century AD. Over the centuries, it has seen revivals, notably in the Renaissance (Neostoicism) and in the modern era (modern Stoicism).
The Stoics provided a unified account of the world, consisting of formal logic, monistic physics and naturalistic ethics. Of these, they emphasized ethics as the main focus of human knowledge, though their logical theories were of more interest for later philosophers.
Stoicism teaches the development of self-control and fortitude as a means of overcoming destructive emotions; the philosophy holds that becoming a clear and unbiased thinker allows one to understand the universal reason (logos). A primary aspect of Stoicism involves improving the individual's ethical and moral well-being: "Virtue consists in a will that is in agreement with Nature." This principle also applies to the realm of interpersonal relationships; "to be free from anger, envy, and jealousy," and to accept even slaves as "equals of other men, because all men alike are products of nature".