Posterior AnalyticsAristotle

The Posterior Analytics is Aristotle's account of what scientific knowledge is and how it is acquired. To know something scientifically is to know its cause and to know that it cannot be otherwise. This is demonstration: a syllogism whose premises are true, primary, immediate, better known than, prior to, and explanatory of the conclusion.

Book I works out the structure of demonstrative science. Every demonstration begins from first principles that are themselves indemonstrable. These principles differ by science; geometry starts from different axioms than medicine. Aristotle distinguishes axioms common to all sciences (like the law of non-contradiction) from hypotheses and definitions proper to each. A science explains by showing that its conclusions follow necessarily from these starting points.

Book II asks how we arrive at the definitions and first principles that demonstrations require. Demonstration cannot prove its own premises, so another capacity is needed. Aristotle traces a path from perception to memory, from memory to experience, and from experience to the grasp of universals. The final step, which he calls nous (intuition or intellect), is not a deduction but a direct apprehension of the universal in the particular. This is induction in its strongest sense: not enumeration of cases, but insight into the intelligible structure that the cases display.

The Posterior Analytics set the terms for every subsequent debate about the nature of scientific explanation, the role of axioms, and the relation between experience and understanding.