Democritus, answered from the texts and cited to the page.
The ethics of euthymia is grounded in the atomic physics of the soul. The soul is fine spherical atoms — the most mobile and most easily disturbed of atomic configurations, distributed through the body and concentrated in particular organs.1 The well-ordered soul is one whose atomic configuration is stable, undisturbed, in equilibrium; euthymia (cheerfulness, "good-spiritedness") names this equilibrium-state.
The disturbed soul — agitated by excessive desires, fortune-chasing, fear, anger — is one whose atomic configuration is violently perturbed. The ethical advice (D27: "must not do many things"; D32: "accustomed to derive his joys from himself"; D29: fortune as excuse for folly) is the practical advice for maintaining the atomic stability of the soul.
The connection is not metaphorical. The ethics flows from the physics. Vlastos in his classic 1945-46 Philosophical Review study made the case for the integration with characteristic rigor; Furley in Two Studies in the Greek Atomists (1967) developed it further. The "laughing philosopher" tradition that emphasizes ironic distance from human folly is real but secondary; the primary content is the systematic naturalistic ethics that follows from the atomic theory of the soul.
The Democritean ethics is the first systematic naturalistic ethics in Western thought, and it is naturalistic precisely because it is integrated with the atomic-physical understanding of the soul as a configuration that can be more or less well-ordered.2
And souls which change over great intervals are neither stable nor cheerful,' but, as I argue in Taylor 114 (p. 13), it is 233 Doctrines: Ethics and Politics quite implausible, given the Democritean picture of the soul as a structure of particularly mobile atoms, to interpret 'change over great intervals' as a description of a state of physical motion.The Atomists, Leucippus and Democritus, p. 220
I epirusmie having the from the epirreo. That is the of the is in this rusmos (an Ionic m of ruthmos) for of Democritus (40 is Peri Ameipsirusmidn On of ameipsirusmie is a. the noun epirusmie is not epirruthmizein in the of therefore interpret the word here as a noun, ameipsirusmie, the sense is a ameipsirusmie.) 13 Ethics: O r Writers D22. re are two forms judgment, genu e and bastard.The Atomists, Leucippus and Democritus, pp. 19–20
The relation between the Democritean ethics and the atomic physics has been contested since antiquity. Vlastos's two-part study in Philosophical Review (1945-46) was the major modern defense of the integration thesis: ethics flows from physics through the atomic theory of soul. Furley (1967) developed the reading further. The alternative view treats the ethics as a separate project running parallel to but independent of the physics (a position closer to Bailey 1928); on this reading the aphoristic-ethical fragments stand on their own as practical wisdom, structurally autonomous from the atomic theory. A third view treats the ethics as inherited popular wisdom in the gnomic tradition of the early sages (Solon, Bias), with the philosophical innovation residing entirely in the physics. The "laughing philosopher" tradition of Cicero and Seneca is generally treated as late legend rather than substantive doctrine. Taylor (1999) takes a careful intermediate position acknowledging the integration thesis while maintaining that the surviving fragments do not always make the connection explicit. The Epicurean systematization of the physics-ethics connection (in clinamen, in the role of prolēpsis, in the explicit derivation of ataraxia from atomic theory) was the systematic development of what Democritus had in less systematic form.