Aristotle formalized in the Organon (c. 350 BC) the first complete system of deductive logic in history. The Prior Analytics establishes syllogistic theory: every valid inference can be reduced to a structure of three propositions (major premise, minor premise, conclusion) in which the conclusion necessarily follows from the premises. The Posterior Analytics distinguishes scientific demonstration from mere persuasion, establishing that genuine knowledge requires deduction from first principles. The Categories classify the ways in which something can be predicated of something else, founding ontology as a discipline. The Aristotelian system is the direct formalization of the Socratic method of questioning and refutation, to which it gives explicit propositional structure. It remains the only system of formal logic until Boole and Frege in the 19th century.