The two operations of our understanding, intuition and deduction, on which alone we have said we must rely in the acquisition of knowledge.

This sentence comes from Descartes’ early methodological writings in which he tries to identify the basic mental acts that can yield certain knowledge. In the 1620s–1630s, reacting against scholastic reliance on authority and disputation, Descartes argues that genuine science must proceed from what the mind can grasp with clarity and certainty. In the Rules for the Direction of the Mind (written c. 1628, published posthumously), he reduces reliable cognition to two operations: “intuition” (the mind’s immediate, clear apprehension of simple truths) and “deduction” (the necessary inference of one truth from others). The remark appears in his program for a rigorous method modeled on mathematics.

Descartes is claiming that all secure knowledge rests on two and only two intellectual acts. “Intuition” is not a hunch but an immediate, unmistakable grasp of a simple proposition (e.g., that a triangle has three sides, or that one is thinking). “Deduction” is the step-by-step, necessity-governed movement from such clear starting points to further conclusions. By limiting epistemic trust to these operations, Descartes excludes testimony, tradition, and sensory impressions unless they can be reconstructed into clear intuitions and valid deductions. The line encapsulates his rationalist ambition: to rebuild the sciences on foundations as certain as geometry.