What does he mean when Epictetus says a person should live in accordance with nature?
Epictetus, a Greek Stoic philosopher, defines nature as (1) that which is "invariable, inviolable, and valid for all human beings"; (2) divided into prohairetic things and aprohairetic things; (3) consisting of the concepts of Prohaireses and Dihaairesis. His definition of the nature of things is exemplified in his statements: "I must die. Must I then die lamenting? ... Does any man then hinder me from going with smiles and cheerfulness and contentment?" and "O Crito, if it thus pleases the gods, thus let it be."
First of all, the nature of things, as inviolable, is objective and the same for all; it is not subject as some claimed. Within this inviolable reality, there are things of the prohairetic category that our choices and volition cannot change, which for Epictetus includes judgements in a court of law or a king's decree ("I must be put in chains. Must I then also lament? I must go into exile. Does any man then hinder me ... from cheerfulness ...?"). On the other hand, there are things of the aprophairetic category that our choices and volition can change (e.g., health). He says of the prohairetic category, which our wills cannot change, that these are not subject to what is called our exclusive power and that it includes aversion, judgment, desire, etc. He says of the aprophairetic category, which our wills can change, that these are subject to what is called our exclusive power and that it includes wealth, health, and fame etc.; these things in this category are called adiaphora.
Secondly, the concept of Prohairesis encompasses our judgments and causes our feeling of attraction or repulsion such as desire or aversion, impelling or repelling, assent or dissent. On the other hand, Dihairesis is a function performed by Prohairesis and is the function through which we recognize or discern that which falls under the sway of and is subject to our exclusive power and that which is not subject to our exclusive power. Thus to know and live in accord with the nature of things means to recognize that the nature of things is (1) objective and inviolable; (2) either prohaietic, thus not subject to our exclusive power or aprophairetic and thus subject to our exclusive power; (3) perceived through Prohairesis and acted upon according to Dihairesis.