The notion of kathêkon is crucial within Stoic ethics. It describes an act which is appropriate for certain reasons. However, the conception behind this word is hard to handle. This can easily be seen by the attempts of translation. They range from “duty” which suggests a highly moral conception to “proper function” which appears almost ethically neutral. This illustrates a particular difficulty, namely that kathêkon represents a conception which is not exclusively ethical but also somehow related to natural science, or what the Stoics call physics.

In my presentation, I propose that the conception of kathêkon is even deeply rooted in physics. This seems to be the result of tracing back the notion to its origin. I shall demonstrate this by analyzing  the definition of kathêkon as it is ascribed to the Early Stoa (DL 7.107.8-10). The only way of reading this definition consistently is with regard to physics. I will show that the vocabulary can reassure such a reading if one takes into account especially Aristotle’s writings on natural science.