Aristotle

Aristotle (384–322 BCE) was a student of Plato, tutor of Alexander the Great, and one of the most comprehensive and influential thinkers who has ever lived. His writings — covering Logic, biology, Metaphysics, physics, Ethics, Aesthetics, rhetoric, and Political Philosophy — shaped the intellectual tradition of Europe, the Islamic world, and beyond for two millennia.

Life

Born in Stagira in northern Greece, Aristotle came to Athens at seventeen to study at Plato's Academy, where he remained for twenty years. After Plato's death he left Athens, tutored the young Alexander (later "the Great"), and in 335 BCE founded his own school in Athens: the Lyceum. His followers were called Peripatetics (from the Greek for "walking about"). He fled Athens in 323 BCE after Alexander's death, reportedly saying he would not allow Athens to "sin twice against philosophy" (alluding to Socrates' execution). He died the following year.

Logic

Aristotle invented formal logic. His Organon ("instrument") contains:

Categories — classification of beings
Prior Analytics — syllogistic logic
Posterior Analytics — scientific demonstration
Topics and Sophistical Refutations — dialectical and fallacious reasoning

See Logic for the full treatment of his contribution.

Metaphysics

Aristotle rejected Plato's Theory of Forms, arguing that universals do not exist separately from particulars:

Substance (ousia) is the primary category of being — individual things (this horse, this human)
Form and matter are inseparable: the form of a thing is what makes it the kind of thing it is; matter is what it is made of
The four causes explain why anything is what it is: material, formal, efficient, final
God (the Unmoved Mover) is pure form without matter — the final cause of all motion

See Metaphysics for comparison with other positions.

Ethics: The Good Life

Aristotle's Ethics is developed in the Nicomachean Ethics. Its central concept is eudaimonia — usually translated as "happiness" or "flourishing":

The good life is one of activity in accordance with virtue
Virtue (arete) is a mean between two extremes: courage is the mean between cowardice and recklessness
The highest good is the contemplative life (theoria) — the life of the philosopher

See Virtue Ethics for a full account of Aristotle's ethical theory.

Political Philosophy

For Aristotle, humans are political animals (zoon politikon) — we are naturally suited to life in a polis (city-state). In the Politics:

The polis exists for the sake of the good life, not merely survival
Constitutions can be good (monarchy, aristocracy, polity) or corrupt forms (tyranny, oligarchy, democracy)
The best realistic constitution is a mixed polity combining elements of oligarchy and democracy
Slavery and the subjugation of women were, regrettably, defended as natural

See Political Philosophy for the broader context.

Aesthetics

In the Poetics, Aristotle defended art against Plato's suspicions:

Mimesis (imitation) is natural and educational
Tragedy produces katharsis — a clarification and purgation of pity and fear
The best plots involve peripeteia (reversal) and anagnorisis (recognition)

See Aesthetics for comparison with Plato and Kant.

Natural Science

Aristotle was the first systematic empirical naturalist — classifying animals, describing embryology, theorizing about the elements. His physics (including the geocentric cosmos) was overturned by Galileo and Newton, but his biological work was largely confirmed only in the 19th century.

Influence

Aquinas synthesized Aristotle with Christian theology in the 13th century
The Islamic philosophers (Avicenna, Averroes) preserved and developed Aristotle
His logic was not superseded until Frege in the 19th century
His virtue ethics was revived by Alasdair MacIntyre and others in the 20th century

Plato — Aristotle's teacher and chief philosophical opponent
Socrates — the source of the tradition Aristotle inherited
Logic — Aristotle invented formal logic
Metaphysics — Aristotle's substance metaphysics vs. Plato's Forms
Ethics — Aristotle's Nicomachean Ethics
Virtue Ethics — Aristotle as founding figure
Political Philosophy — humans as political animals
Aesthetics — katharsis and the defence of tragedy
The Forms — Aristotle's critique of Plato's theory