Prostheke

The duty or obligation arising from the rational nature of the human being and their social constitution — not as an external prescription but as an inner consequence.

Prostheke (Greek: addition, supplement) refers in Stoicism to the duty that is necessary for the rational and social being by virtue of their nature. It does not arise from convention or obedience to external law, but follows logically from two interconnected facts: that the human being possesses reason (and can therefore recognise what is good) and that they are a social being (and therefore stands in relationship to others). Prostheke is closely related to kathekon (appropriate action), with Prostheke emphasising the qualitative dimension — that which marks an action as a duty.