The Indemonstrables (Stoic Propositional Logic)

Where Aristotle's logic reasoned about classes of things ("all men," "some animals"), the Stoics built a rival system reasoning about whole statements joined by "if," "and," "or," and "not." At its heart lay five basic argument-forms called the indemonstrables — patterns like "If the first, then the second; but the first; therefore the second" — held to be so obviously valid that every other valid argument could be reduced to them without further proof. Largely forgotten for centuries, this Stoic propositional logic anticipated the modern logic of connectives by nearly two thousand years.