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Ataraxia (ἀταραξία, "not perturbed") is a Greek term used by Pyrrho and subsequently Epicurus for a lucid state of robust equanimity, characterized by ongoing freedom from distress and worry. The ancient Greek author Sextus Empiricus gave this definition: "ataraxia is an untroubled and tranquil condition of the soul."  In non-philosophical usage, the term was used to describe the ideal mental state for soldiers entering battle.
For Epicureanism, ataraxia was synonymous with the only true happiness possible for a person. It signifies the state of robust tranquillity that derives from eschewing concerns about 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, realizing that the physical things one needs to be happy are few and that pain seldom lasts long, and, most importantly, being an affectionate, virtuous person, worthy of trust.
For Pyrrhonism, Sextus Empiricus said "For the person who believes that something is by nature good or bad is constantly upset; when he does not possess the things that seem to be good, he thinks he is being tormented by things that are by nature bad, and he chases after the things he supposes to be good; then, when he gets these, he falls into still more torments because of irrational and immoderate exultation, and, fearing any change, he does absolutely everything in order not to lose the things that seem to him good. But the person who takes no position as to what is by nature good or bad neither avoids nor pursues intensely. As a result, he achieves ataraxia."
Stoicism often made use of the term, as they too sought mental tranquillity and saw ataraxia as highly valuable. In Stoicism, however, ataraxia is not an end to be pursued for its own sake, but is rather a natural consequence that occurs in a person who pursues virtue. A closely related state, attained by the ideal Stoic sage, was the absence of unhealthy passions, or apatheia.
- Sextus Empiricus, "The Skeptic Way", Trans. Benson Mates, Book I, Ch. IV, "What Skepticism Is", p. 3
- Sextus Empiricus, "The Skeptic Way", Trans. Benson Mates, Book I, Ch. XII, "What Is the Goal of Skepticism?", p. 6
- Steven K. Strange, (2004), The Stoics on the Voluntariness of Passion in Stoicism: Traditions and Transformations, page 37. Cambridge University Press.