Physis

The nature of a thing or being; the inner essence that determines how it ought to live in accordance with its kind.

Physis (φύσις) in Stoic philosophy denotes not merely external, material nature, but above all the inner, characteristic nature or essence of a thing. This nature is rationally structured through the logos and shapes what is "natural" or "appropriate" for a given thing. In the human being, physis is the rational character; to live well means to live in accordance with this nature. The Stoics distinguish individual physis from universal physis (the cosmic Logos).