The senses deceive from time to time, and it is prudent never to trust wholly those who have deceived us even once” is a quote from René Descartes, a French philosopher known for his skepticism about the reliability of the senses.
The first part of the quote, “The senses deceive from time to time,” refers to Descartes’ belief that our senses are not always reliable sources of truth. He gives examples of optical illusions and dreams as instances where our senses give us inaccurate representations of the world. For instance, a stick may appear bent when it’s half-submerged in water, or a square tower might look round from a distance. In a dream, one might have sensations that feel real but aren’t actually happening.
The second part of the quote, “It is prudent never to trust wholly those who have deceived us even once,” is a sort of philosophical advice from Descartes. He extends the unreliability of the senses to the concept of trust in general. His perspective is that if someone (or something, such as our senses) has deceived us once, it’s wise to maintain a degree of skepticism toward them in the future because there’s a demonstrated potential for deception.
In the broader context of Descartes’ philosophy, this quote underpins his method of systematic doubt, which was a foundational tool in his philosophical method. He decided to doubt everything he thought he knew, to identify those beliefs that were absolutely certain. This is how he arrived at his famous dictum, “Cogito, ergo sum” (“I think, therefore I am”), which is the first principle of his philosophy, and the one thing he believed couldn’t be doubted.