Aristotle & the Science of Logic

Aristotle's first great departure from Plato was his rejection of the Theory of Forms. He found the idea of a separate realm of Forms both metaphysically extravagant and explanatorily useless. His argument — later sharpened by others — runs roughly as follows: If there is a Form of Man that makes all individual men men, then there must be something that makes individual men and the Form of Man related — a third thing that relates the two. But then there must be a fourth thing that relates all three, and so on ad infinitum. This is the "Third Man Argument," and it threatens an infinite regress of Forms.

Aristotle's alternative was immanent universals: universals do not exist in a separate realm but in the particular things that instantiate them. The form of a horse is not in a separate Platonic heaven; it is in the horse itself, inseparable from the horse's matter. When the horse dies, that particular instantiation of horseness ceases to exist — but horseness can still be known through encounter with other horses.

This difference is not merely technical. It reflects a fundamental difference in temperament. Plato was a geometer who trusted mathematical abstraction over sensory experience. Aristotle was a biologist who trusted careful observation of the natural world. His philosophy is correspondingly more attentive to change, growth, development, and the particular — things that are difficult to accommodate in a world of eternal, unchanging Forms.

Aristotle's single most influential contribution to intellectual history may be his invention of formal logic — the systematic study of valid inference. Before Aristotle, arguments were evaluated case by case. He was the first to extract the form of an argument from its content and evaluate the form independently. A syllogism like "All men are mortal; Socrates is a man; therefore Socrates is mortal" is valid not because of anything special about men or Socrates, but because of the form "All A are B; C is an A; therefore C is B."

This was a revolutionary insight. It meant that validity could be assessed without considering whether the premises were actually true — that logic and epistemology were separate disciplines. It also provided a standard for evaluating arguments across all domains, from mathematics to rhetoric to theology. Aristotle's logical works — the collection called the Organon ("Instrument") — remained the dominant framework for logic until the 19th century, when Frege and Russell developed modern predicate logic.

In the Nicomachean Ethics — addressed to or named after his son Nicomachus — Aristotle developed the most influential ethical theory of the ancient world: virtue ethics. His starting point is teleological: everything has a characteristic function (ergon), and the good for a thing is the excellent performance of its function. A good knife cuts well; a good eye sees well; a good human being...what?

Aristotle's answer: a good human being reasons and acts well. This is because the distinctive function of human beings — what separates us from plants and non-rational animals — is our capacity for rational activity. The good life (eudaimonia, often translated "happiness" but better rendered "flourishing" or "living well") consists in the excellent exercise of this capacity over a complete lifetime.

Virtues are the stable character traits — courage, justice, temperance, practical wisdom (phronesis) — that enable us to reason and act well. Each virtue is a mean between two extremes of vice: courage is the mean between cowardice and recklessness; generosity is the mean between miserliness and prodigality. Crucially, virtues are not innate; they are habits acquired through practice. You become courageous by doing courageous things; you become just by performing just acts. The ethical life is not a matter of following rules but of developing character through sustained practice.