Massimo Pigliucci in New Humanist:
Let me then introduce you to three fundamental ideas of Stoicism – one theoretical, the other two practical – to explain why I’ve become what I call a secular Stoic. To begin with, the Stoics – a school of philosophers who flourished in the Greek and Roman worlds for several hundred years from the third century BCE – thought that, in order to figure out how to live our lives (what they called ethics), we need to study two other topics: physics and logic. “Physics” meant an understanding of the world, as best as human beings can grasp it, which is done by way of all the natural sciences as well as by metaphysics. The reason that physics is considered so important is that attempting to live while adopting grossly incorrect notions about how the world works is a recipe for disaster. “Logic” meant not only formal reasoning, but also what we would today call cognitive science: if we don’t know how to use our mind correctly, including an awareness of its pitfalls, then we are not going to be in a position to live a good life.
The ancient Stoics explained the idea by way of a metaphor introduced by Chrysippus of Soli, the second head of the Stoa, as the Stoic school was known. It was named after the stoa poikile, the painted porch in Athens, a public place where Stoics would gather to discuss philosophy with whoever was interested. According to the metaphor, a life worth living is like a fenced garden: the fence itself is logic, as it guards the inside from weeds and other noxious things; the nurturing soil is the physics, since it informs us on how to navigate the world to the best of our abilities; and the fruits are the ethics, resulting in a eudaimonic (happy or flourishing) life, the sort of life that one looks back to on her death bed and thinks, “Yup, that was pretty well done.” This means that Stoic theory embraces the humanist emphasis on an ethical life, but also directly justifies our interests in both metaphysics and natural sciences (“physics”) as well as philosophy and social science (“logic”). They all come together in a satisfyingly coherent package.
The first practical notion I’m going to discuss is that of the three disciplines and their related four virtues. Epictetus, a slave who became one of the most influential teachers of antiquity, thought that there are three areas of application of Stoic philosophy – what are now known as the three disciplines: desire, action and assent.