The Unreliability of Sense Perception

Descartes did not believe that the information we receive through our senses is necessarily accurate. After the revelation he experienced on November 10, 1619, Descartes undertook his own intellectual rebirth. His first step was to throw out everything he thought he knew, refusing to believe in even the most basic premises before proving them to himself satisfactorily. In this act of demolition and reconstruction, Descartes felt it would be a waste of time to tear down each idea individually. Instead, he attacked what he considered the very foundation: the idea that sense perception conveys accurate information. He developed several arguments to illustrate this point.

In the Dream argument, Descartes argues that he often dreams of things that seem real to him while he is asleep. In one dream, he sits by a fire in his room, and it seems he can feel the warmth of the fire, just as he feels it in his waking life, even though there is no fire. The fact that he feels the fire doesn’t really allow him to tell when he is awake and when he is dreaming. Moreover, if his senses can convey to him the heat of the fire when he does not really feel it, he can’t trust that the fire exists when he feels it in his waking life.

Likewise, in the Deceiving God and Evil Demon arguments, Descartes suggests that, for all he knows, he may be under the control of an all-powerful being bent on deceiving him. In that case, he does not have a body at all but is merely a brain fed information and illusions by the all-powerful being. (Fans of the Matrix films may recognize this concept.) Descartes does not intend these arguments to be taken literally. His point is to demonstrate that the senses can be deceived. If we cannot trust our senses to convey true information about the world around us, then we also can’t trust deductions we’ve made on the grounds of sense perception.

At the time Descartes cast doubt on the reliability of sense perception, it was a radical position. He was proposing that scientific observation had to be an interpretive act requiring careful monitoring. The proponents of the British empiricist movement especially opposed Descartes’ ideas. They believed that all knowledge comes to us through the senses. Descartes and his followers argued the opposite, that true knowledge comes only through the application of pure reason.

Science Based on Reason

Although Descartes mistrusted the information received through the senses, he did believe that certain knowledge can be acquired by other means, arguing that the strict application of reason to all problems is the only way to achieve certainty in science. In Rules for the Direction of the Mind, Descartes argues that all problems should be broken up into their simplest parts and that problems can be expressed as abstract equations. Descartes hopes to minimize or remove the role of unreliable sense perception in the sciences. If all problems are reduced to their least sense-dependent and most abstract elements, then objective reason can be put to work to solve the problem.

Descartes’ work combining algebra and geometry is an application of this principle. By creating a two-dimensional graph on which problems could be plotted, he developed a visual vocabulary for arithmetic and algebraic ideas. In other words, he made it possible to express mathematics and algebra in geometric forms. He also developed a method to understanding the properties of objects in the real world by reducing their shapes to formulae and approaching them through reason rather than sense perception.

Reason as the Essence of Humanity

Descartes’ most famous statement is Cogito ergo sum, “I think, therefore I exist.” With this argument, Descartes proposes that the very act of thinking offers a proof of individual human existence. Because thoughts must have a source, there must be an “I” that exists to do the thinking. In arguments that follow from this premise, Descartes points out that although he can be sure of nothing else about his existence—he can’t prove beyond a doubt that he has hands or hair or a body—he is certain that he has thoughts and the ability to use reason. Descartes asserts that these facts come to him as “clear and distinct perceptions.” He argues that anything that can be observed through clear and distinct perceptions is part of the essence of what is observed. Thought and reason, because they are clearly perceived, must be the essence of humanity. Consequently, Descartes asserts that a human would still be a human without hands or hair or a face. He also asserts that other things that are not human may have hair, hands, or faces, but a human would not be a human without reason, and only humans possess the ability to reason.

The Attainability of Knowledge

Descartes firmly believed that reason is a native gift of humans and that true knowledge can be directly gleaned not from books but only through the methodical application of reason. The expressed aim of many of his books was to present complex scientific and philosophical matters in such a way that the least sophisticated readers could understand them. Because Descartes believed that every human possesses the “natural light” of reason, he believed that if he presented all his arguments as logical trains of thought, then anyone could understand them and nobody could help but be swayed. In the original edition of Discourse on the Method, in fact, Descartes declares his aim with the subtitle “In which the Author… explains the most abstruse Topics he could choose, and does so in such a way that even persons who have never studied can understand them.” In an attempt to reach a wider audience, Descartes occasionally wrote in French, the language of his countrymen, rather than Latin, the language of scholars, so that people without a formal education could understand him.