Port-Royal Logic

Port-Royal Logic, or Logique de Port-Royal, is the common name of La logique, ou l'art de penser, an important textbook on logic first published anonymously in 1662 by Antoine Arnauld and Pierre Nicole, two prominent members of the Jansenist movement, centered on Port-Royal. Blaise Pascal likely contributed considerable portions of the text. Its linguistic companion piece is the Port-Royal Grammar (1660) by Arnauld and Lancelot.

The book helped popularize the concept that a definition with more qualifications or features (the intension) denotes a class with fewer members (the extension), and vice versa. It was based on Aristotle's ideas about genus and species. The same concept would later become a fundamental element in the works of Gottfried Wilhelm Leibniz.