The senses deceive from time to time, and it is prudent never to trust wholly those who have deceived us even once.

Descartes introduces this thought at the outset of his project of methodological doubt, where he resolves to suspend belief in anything that can be called into question. Writing in the early 1640s, amid debates about scholastic Aristotelianism and the foundations of the new science, he argues that much of what he has accepted as true came through the senses—yet the senses sometimes mislead (as in perceptual illusions or distant objects). This observation becomes an initial, commonsense motive for doubting sensory-based beliefs, clearing the ground for seeking a more secure foundation for knowledge.

The remark is not a blanket denial of sensory experience so much as a caution about its epistemic authority. Because the senses have occasionally produced error, Descartes treats them as an unreliable witness when the goal is certainty. The line encapsulates a key move in his method: to rebuild knowledge, one must begin by withholding assent from sources that admit any doubt. It also signals a shift from trusting inherited or empirical appearances to demanding clear and indubitable justification—preparing the way for his later claims that some truths (like the cogito) are known more securely than anything delivered by perception.