Apatheia (Greek: ἀπάθεια; from a- "without" and pathos "suffering" or "passion"), in Stoicism, refers to a state of mind in which one is not disturbed by the passions. It might better be translated by the word equanimity than the word indifference. The meaning of the word apatheia is quite different from that of the modern English apathy, which has a distinctly negative connotation. According to the Stoics, apatheia was the quality that characterized the sage.
Whereas Aristotle had claimed that virtue was to be found in the golden mean between an excess and a deficiency of emotion (metriopatheia), the Stoics thought that living virtuously provided freedom from the passions, resulting in apatheia. It meant eradicating the tendency to react emotionally or egotistically to external events, the things that cannot be controlled. For Stoics, it was the optimally rational response to the world, for things cannot be controlled if they are caused by the will of others or by Nature; only one's own will can be controlled. That did not mean a loss of feeling, or total disengagement from the world. The Stoic who performs correct (virtuous) judgments and actions as part of the world order experiences contentment (eudaimonia) and good feelings (eupatheia).
Pain is slight if opinion has added nothing to it;... in thinking it slight, you will make it slight. Everything depends on opinion; ambition, luxury, greed, hark back to opinion. It is according to opinion that we suffer.... So let us also win the way to victory in all our struggles, – for the reward is... virtue, steadfastness of soul, and a peace that is won for all time.— Seneca,Epistles, lxxviii. 13–16
The term was later adopted by Plotinus in his development of Neoplatonism, in which apatheia was the soul's freedom from emotion achieved when it reaches its purified state. It passed into early Christian teaching in which apatheia meant freedom from unruly urges or compulsions. It is still used in that sense in Orthodox Christian spirituality, and especially in monastic practice.
Apatheia is contrasted with ataraxia, a related concept in Epicureanism and Pyrrhonism, although some Latin Stoic authors, such as Seneca the Younger use the term interchangeably with apatheia. In Epicureanism ataraxia comes from freedom from pain and fear. In Pyrrhonism it comes from the eradication of disturbing feelings that depend on beliefs about non-evident matters (i.e., dogma).