Logic or the Art of Thinking

The Port-Royal Logic (La Logique ou l'Art de Penser) by Antoine Arnauld and Pierre Nicole — the most widely used logic textbook in Europe for over a century, combining traditional syllogistic with probability reasoning.

Introduced probability and degrees of belief into logic; expanded logic beyond deductive inference to include rhetorical and probabilistic reasoning; applied to theological disputes. Shaped French and English intellectual culture for generations.