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.
