Abstract: The doctrine of Aristotles deduction can be divided into demonstrative deduction and non-demonstrative deduction.The core of the latter is an informal reasoning tool named topoi,which provides a universal guidance for the arguer to construct arguments from endoxa.The maturation of such doctrine contributes to the philosophization of rhetoric and makes up for the shortcomings of demonstrative deduction theory in the filed of practical reasoning and the discovery of first principles.It features prominently in Aristotles logic,rhetoric and epistemology.

Key words: Aristotle, non-demonstrative deduction