What makes a life go well? Not an afternoon or a quarter — a life, taken whole and judged as a completed thing. The Greek schools treated this as the organising question of philosophy and everything in ethics as downstream of the answer.

Why the usual translation fails

Happiness in English names a transient feeling, and eudaimonia does not. It is predicated of a life rather than a moment, which is why Aristotle can seriously discuss whether someone's eudaimonia is affected by what happens after their death (Nicomachean Ethics I.10) — an unintelligible question about a mood. Flourishing is the better rendering; Solon's advice in Herodotus, to call no man happy until he is dead, makes sense only on the second reading.

The structure of the claim

Aristotle's argument in Nicomachean Ethics I moves in steps. All action aims at some good. Ends nest — we pursue some things for the sake of others. The chain cannot regress without end, so there is a final end, wanted for its own sake and never for anything further. That is eudaimonia, and naming it settles nothing, since everyone agrees on the word and disagrees on the content.

His answer comes from the function argument. Everything with a characteristic activity has its good in performing that activity well; what is distinctive of humans is rational activity; so the human good is activity of the soul in accordance with virtue, over a complete lifeAristotle, Nicomachean Ethics I.7, 1098a16–18. Two qualifications carry weight: 'activity', not a state — eudaimonia is something done, not possessed — and 'a complete life', which is why a good day cannot constitute it.. Activity rather than state, and over a complete life rather than at a moment. Book X then complicates it by appearing to elevate contemplation above the practical virtues, and whether the two accounts are consistent is a long-running scholarly dispute.

Where the schools divide

The Stoics agreed on the framework and rejected the content: virtue alone is sufficient for eudaimonia, and health, wealth and reputation are indifferents. Aristotle disagreed explicitly, holding that external goods are required — the person on the rack is not flourishing, whatever their virtue. Epicurus identified the good with pleasure, understood as the absence of bodily pain and mental disturbance rather than as gratification, which makes his position far closer to the others than its reputation suggests. What all three share is the claim that the question has an answer determined by human nature rather than by preference.

The objections

The function argument is the weak joint. It moves from what is distinctive of humans to what is good for them, and that inference needs an argument it does not receive — many things are distinctive of a species without being good for its members. This is a version of the naturalistic fallacy, and the modern neo-Aristotelians who revived the position, notably Philippa Foot (2001), had to rebuild the link through an account of natural goodness rather than inherit it.

The second objection is that eudaimonism may be unable to accommodate moral obligation to others except instrumentally — if the aim is my flourishing, sacrifice needs explaining. The standard reply is that on the Greek account friendship and justice are constituents of flourishing rather than instruments to it, which answers the objection if the constitutive claim can be sustained. And a third, from the modern side: cross-cultural work on wellbeing suggests the content of a flourishing life varies more than an account grounded in a single human function can easily allow.

What it rules out

It rules out self-report of mood as a measure of whether a life is going well. It rules out evaluating a period in isolation, since the unit of assessment is a whole life. And it rules out treating the good life as a matter of preference, which is precisely what the modern usage of 'happiness' assumes.

It does not rule out feeling good. Aristotle holds that the virtuous person takes pleasure in virtuous action and that someone who acts well while hating it has not yet arrived — pleasure accompanies flourishing rather than constituting it.

Sources

Aristotle. Nicomachean Ethics I, II, X. · Epicurus. Letter to Menoeceus. · Foot, P. (2001). Natural Goodness. Oxford University Press. · Herodotus. Histories I.30–33. · Hursthouse, R. (1999). On Virtue Ethics. Oxford University Press. · Seneca. De Vita Beata.