Ataraxia

Ataraxia (Ἀταραξία "tranquility") is a Greek term used by Pyrrho and Epicurus for a lucid state, characterized by freedom from worry or any other preoccupation.

For the Epicureans, ataraxia was synonymous with the only true happiness possible for a person. It signifies the state of robust tranquility that derives from eschewing faith in an afterlife, not fearing the gods because they are distant and unconcerned with us, avoiding politics and vexatious people, surrounding oneself with trustworthy and affectionate friends and, most importantly, being an affectionate, virtuous person, worthy of trust.

For the Pyrrhonians, owing to one's inability to say which sense impressions are true and which ones are false, it is the quietude that arises from suspending judgment on dogmatic beliefs or anything non-evident and continuing to inquire. The experience was said to have fallen on the painter Apelles who was trying to paint the foamy saliva of a horse. He was so unsuccessful that, in a rage, he gave up and threw the sponge he was cleaning his brushes with at the medium, thus producing the effect of the horse's foam.

The Stoics, too, sought mental tranquility, and saw ataraxia as something to be desired and often made use of the term, but for them the analogous state, attained by the Stoic sage, was apatheia or absence of passion.