Aristotle’s logical work has been handed down to us as a group of six treatises of variable scope, collected under the title Organon, which means ‘instrument’ or ‘tool’. The collection is opened by the Categories, a short treatise which introduces the ten Aristotelian categories, the classification of the ultimate kinds of being which Aristotle discusses or deploys in virtually all his works, but examines only substance, quantity, relative, and quality, and moves then on to the different kinds of opposition. The second treatise in the traditional order, De Interpretatione (Peri hermēneias, perhaps best translated as On Expression, but normally referred to with its Latin title), deals with language: it defines the name and the verb, and discusses what affirmation and negation are. The third treatise, Prior Analytics, turns to logic in a narrower sense, and offers a thorough and quite technical exposition of deductive arguments – its core is Aristotle’s assertoric syllogistic, the system which has dominated Western logical thought for more than two millennia. A second system, modal syllogistic, is also expounded in detail. The fourth treatise, Posterior Analytics, builds on the theory of syllogism in order to elucidate the notion of demonstration and the structure of a demonstrative science; its second book studies definition and the principles of science. The fifth treatise, the Topics, is a lengthy collection and classification of patterns for dialectical arguments, and the sixth, the Sophistical Refutations, deals with fallacies, i.e. apparently good arguments which are in fact invalid or otherwise defective.1