 Contradiction (tanāquḍ) and conversion (ʿaks) are fundamental relations in Avicenna’s logic. The principles that govern them are central in the development of the syllogistic, both categorical and hypothetical. It is not surprising, in light of the complexity of Avicenna’s analysis of categorical propositions and of his elaborate account of quantified hypotheticals, that his answers to questions such as what contradicts what, what follows from what, and what converts with what all came under close scrutiny in Post-Avicennan logic, before finding a stable equilibrium with Kātibī (d. 1276 CE), an equilibrium that turns in several key respects on the interpretation of these relations.