From eudaimonia as excellent activity and the calculus of pleasure to the tranquil mind, the meaningful life, and what science can measure.
What is the good life — pleasure, virtue, tranquillity, or the full flourishing of a human being?
Ask what happiness is and the ancient answers diverge sharply from the modern feeling of the word. For the Greeks 'eudaimonia' meant a life well lived as a whole, not a passing mood — and they disagreed over whether it lay in virtuous activity, in pleasure carefully understood, or in freedom from disturbance. The utilitarians turned happiness into a quantity to be maximized, while later thinkers doubted that contentment is even the point of a human life. Today psychology tries to measure well-being empirically, distinguishing momentary pleasure from a life experienced as meaningful.
Ancient Greek (Peripatetic)
Happiness (eudaimonia) is not a feeling but 'activity of the soul in accordance with virtue' over a complete life. It is the final end we seek for its own sake, requiring the cultivation of both moral and intellectual excellences, and needing some external goods and good fortune to be fully realized.
Nicomachean Ethics, Books I and X.
Epicureanism
The goal of life is pleasure, understood as the absence of bodily pain (aponia) and mental disturbance (ataraxia), not sensual indulgence. The wise pursue simple, natural pleasures and, through reasoning, free themselves from the two great fears — of the gods and of death, since 'death is nothing to us'.
Letter to Menoeceus; Principal Doctrines.
Roman Stoicism
For the Stoics, happiness is a smoothly flowing life achieved by living in agreement with nature and reason. It depends solely on virtue and on distinguishing what is 'up to us' from what is not; peace comes from desiring only what is within our power and accepting the rest as it happens.
Enchiridion; Discourses.
Utilitarianism
Happiness is pleasure and the absence of pain, and the right action is the one that promotes the greatest happiness of the greatest number. Refining Bentham, Mill insists pleasures differ in quality as well as quantity — 'better to be Socrates dissatisfied than a fool satisfied' — so the higher, intellectual pleasures count for more.
Utilitarianism (1863).
Existential critique
Nietzsche is suspicious of comfortable happiness as an ideal: 'Man does not strive for happiness; only the Englishman does.' What matters is not contentment but the growth of power, creativity and self-overcoming; a great life may embrace suffering, and one should be able to affirm one's whole existence — 'amor fati' — even to will its eternal recurrence.
Twilight of the Idols (1889); The Gay Science (1882).
Analytic philosophy / secular humanism
Russell held that much unhappiness stems from mistaken worldviews and excessive self-absorption. Happiness is largely to be won by cultivating outward-directed interests and affections — zest for life, work, impersonal enthusiasms — that connect us to the world rather than trapping us in the self.
The Conquest of Happiness (1930).
Behavioural science / psychology
Kahneman distinguishes the 'experiencing self', which lives in the present moment, from the 'remembering self', which judges life in retrospect and dominates our decisions. Because memory is shaped by peaks and endings rather than duration, what we call happiness depends on which self we ask — a distinction that unsettles simple measures of well-being.
Thinking, Fast and Slow (2011); research on the 'peak–end rule'.
Positive psychology
Founder of positive psychology, Seligman argues that well-being is not one thing but several measurable elements — captured in the PERMA model: positive emotion, engagement, relationships, meaning and accomplishment. Lasting flourishing comes less from fleeting pleasure than from engagement, meaning and using one's character strengths.
Authentic Happiness (2002); Flourish (2011).