Euthymia (Greek: εὐθυμία, "gladness, good mood, serenity"—literally "good thumos") is a central concept in the moral thoughts of Democritus, who presents it as an ideal disposition of mind corresponding to a form of equanimity, a calm affectivity and relative steadiness of the soul.
Diogenes Laërtius records Democritus' view as follows: "The chief good he asserts to be cheerfulness (euthymia); which, however, he does not consider the same as pleasure; as some people, who have misunderstood him, have fancied that he meant; but he understands by cheerfulness, a condition according to which the soul lives calmly and steadily, being disturbed by no fear, or superstition, or other passion.
[2] Seneca's dialogue is intented as a remedy for the absence of euthymia of his friend Serenus, who asks him for help: [Serenus:] Not to multiply examples, I am in all things attended by this weakness of a well-meaning mind, to whose level I fear that I shall be gradually brought down, or what is even more worrying, that I may always hang as though about to fall, and that there may be more the matter with me than I myself perceive: for we take a friendly view of our own private affairs, and partiality always obscures our judgment.
[3]Serenus' problem is that, despite recognising the validity of the Stoic principles by which he lives his life, his strength to follow them sometimes wavers.
[tranquillitas][4]Seneca's euthymia, or tranquillity, must not be confused with the idea of self-confidence popularised by self-help literature, as it seems to be the case of influencer Ryan Holiday, who misquotes (indicating no reference) Seneca's work On the tranquility of the mind, wrongly defining euthymia as "believing in yourself and trusting that you are on the right path, and not being in doubt by following the myriad footpaths of those wandering in every direction".