Apatheia (Greek: ἀπάθεια; from a- "without" and pathos "suffering" or "passion") in Stoic philosophy refers to a state of mind where one is not disturbed by the passions. It is best translated by the word equanimity rather than indifference. The word apatheia has a quite different meaning to the modern English apathy, which has a 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 excess and deficiency of emotion (metriopatheia), the Stoics sought freedom from all passions (apatheia). It meant eradicating the tendency to react emotionally or egotistically to external events – the things we cannot control. For the Stoics, it was the optimum rational response to the world, for we cannot control things that are caused by the will of others or by Nature; we can only control our own will. This 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.
The Pyrrhonian skeptics also sought the eradication of feelings when disturbance depends on belief, but allowed for only a moderation of feeling when based on sensations such as pain. 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.