Aretē is one of the most capacious words in Greek ethics. Etymologically it is linked with ἄριστος ("best") and ἀρέσκω ("to please"); its Latin equivalent is virtus. In ordinary Greek it means "the excellence" of anything whatever: the aretē of a knife is sharpness, the aretē of a horse is speed and strength, the aretē of a soldier is courage, the aretē of a human being is what makes him good as a human. Thus aretē = functional excellence in accordance with one's own nature.

The Homeric aretē is chiefly martial valour and nobility. Gradually — by Plato and Aristotle — the word is narrowed to the ethical sphere: human aretē is a state of the soul that makes it perfect with respect to its rational nature.

For the Stoics aretē receives its radical formulation: virtue is the only good (ἡ ἀρετὴ μόνη ἀγαθόν, SVF III 30, 38, 49 and onward). Not one of many goods, not the "highest" good among many, but the only one — everything else (health, wealth, country, offspring, reputation) belongs to the indifferents. The Stoic aretē is technically defined as:

• διάθεσις τοῦ TERMἡγεμονικοῦ κατὰ λόγον — "the disposition of the ruling part in accordance with reason";
• τέχνη βίου — "the art of living";
• ἐπιστήμη τῶν ἀγαθῶν καὶ κακῶν καὶ οὐδετέρων — "knowledge of the good, the bad, and the indifferent."

The four cardinal aretai are inherited from Plato: φρόνησις (practical wisdom), δικαιοσύνη (justice), ἀνδρεία (courage), σωφροσύνη (temperance). The Stoics elaborate a complex taxonomy of sub-species, but they hold to the thesis of the unity of virtue (μία ἀρετή): whoever possesses one in full possesses all. The reason: the four are different applications of one and the same ἐπιστήμη — knowledge of good and evil.

Plat. Resp. IV 427d–445e (the doctrine of the four virtues); Meno (whether aretē can be taught); Aristot. Eth. Nic. II–VI (aretē as μεσότης, the mean); SVF III 197–261 (the Stoic corpus on the virtues); specifically III 30, 38, 49, 65 (ἀρετὴ μόνη ἀγαθόν); DL VII 90–100; Stob. Ecl. II 58–65 W; Cic. De fin. III; Tusc. IV–V; LS 60, 61. In Marcus, aretē is a constant background: Med. 2.5, 2.11, 2.13, 2.17; 3.6 ("prefer nothing above your own inner aretē"); 5.1; 5.9; 5.34; 7.74; 8.32; 10.16; 11.10.

In the Stoic system aretē is not merely a moral quality but the self-sufficient and only value; the whole edifice of ethics is built around it. Connections:

• With TERMτὰ ἀγαθά: aretē is the only genuine bearer of the status "good."
• With TERMεὐδαιμονία: aretē is the only and sufficient cause (see virtue-is-sufficient).
• With TERMἡγεμονικόν: aretē is a determinate disposition of the ruling part; it is not "acquired from outside" but is a state of the inner instance.
• With TERMkakia (vice): aretē and κακία are two polar opposite states of the hēgemonikon, and nothing lies between them (the Stoic "uncompromising" ethical binarism: either the sage or the fool).

In 02-13 aretē appears in the formula «τὰ ἐκ θεῶν αἰδέσιμα δι' ἀρετήν» — "the works of the gods are venerable on account of their aretē." Here aretē refers not to human moral virtue but to the excellence of the gods — that is, to the more general sense of "excellence in accordance with nature." The gods, as ontological beings, possess a perfect nature, and so their works are worthy of αἰδώς. This is a typical use of aretē in the broad sense: aretē is not "moral merit" in the modern sense, but excellence with respect to nature — which in the case of a human coincides with moral virtue, and in the case of the gods with their divine nature.

The Latin virtus, in Cicero, calques aretē, and through Seneca enters the Christian tradition of the "virtues" as the ethical category par excellence. The Roman vir ("man") preserves a heroic colouring that the Greek aretē originally had as well.