The six treatises that make up Aristotle’s Organon are the first writings on logic as an independent discipline to appear in Western civilization. The title has been used to refer to the collection since at least the sixth century, but there is no evidence that Aristotle himself referred to the treatises by this name. Aristotle’s word for what today is called formal logic was “analytics.” Traditionally, the treatises have been ordered as follows: Categories, On Interpretation, Prior Analytics, Posterior Analytics, Topics, and On Sophistical Refutations. This order is based on the contents: Categories treats of terms, On Interpretation treats of propositions, Prior Analytics treats of syllogisms. The remaining three treat of kinds of argument; Posterior Analytics of apodictic (necessary) syllogisms, Topics of dialectical (debatable) syllogisms, and On Sophistical Refutations of unsound arguments (informal fallacies). However, Aristotle did not write the treatises in this order, and there is no evidence to support the rather common misconception that Aristotle regarded them (except for the Prior and Posterior Analytics) as successive chapters in a systematic treatise on logic. The Categories, Topics, and On Sophistical Refutations are early works, On Interpretation was probably written some time later, and the two Analytics were written last. The Categories is perhaps as much a work on metaphysics as it is on logic; it has considerable historical significance, but its logical content is rather meager.
There is a wealth of material discussed in the six works, but it is of very uneven importance. Large portions are tedious and out of date, while other sections are first-rate philosophy and surprisingly modern. What follows is a very brief summary of the contents of each treatise, with a somewhat more detailed account of the Categories and the two Analytics.