One of the primary representatives of this kind of skepticism about the external world was René Descartes, who is often regarded as the first modern philosopher for his break with the scholastic, medieval tradition that preceded him. Descartes was a mathematician, and wanted to find a basis for truth that was as reliable as the mathematical truths of geometry. In his famous book, Meditations on First Philosophy he searches for a founding principle upon which a theory of truth can be based in the way that mathematical truths are all based on a smaller set of foundations like calculus. He hoped to establish a sound basis for scientific method and prove that the real source of truth and scientific knowledge was in the mind, and not in the external senses.In order to show that science rested on firm foundations and that these foundations lay in the mind and not the senses, Descartes began by bringing into doubt all the beliefs that come to us from the senses. His aim in these arguments is not really to prove that nothing exists or that it is impossible for us to know if anything exists (he will prove that we can know external objects later), but to show that all our knowledge of these things through the senses is open to doubt. If our scientific knowledge came to us through the senses, we could not even be sure that anything outside of us existed. The obvious implication is that, since we do know that external objects exist, this knowledge cannot come to us through the senses, but through the mind. Commenting on his own method, Descartes remarked:
“Throughout my writings I have made it clear that my method imitates that of the architect. When an architect wants to build a house which is stable on ground where there is a sandy topsoil over underlying rock, or clay, or some other firm base, he begins by digging out a set of trenches from which he removes the sand, and anything resting on or mixed in with the sand, so that he can lay his foundations on firm soil. In the same way, I began by taking everything that was doubtful and throwing it out, like sand …”
Replies 7, Oeuvres de Descartes, Adam, Charles, and Paul Tannery, (eds.) 1904. Paris: J. Vrin Vol. 7 p.537
So, for Descartes, the doubting even our most everyday beliefs like the external world was a way of ensuring that our general beliefs have a strong epistemological foundation. Descartes uses two famous arguments to open all our knowledge to doubt: the dream argument (It is possible that I am dreaming right now and that all of my perceptions are false), and the evil demon argument (there could exist an evil demon who has it in his power to make us believe in false things, even with respect to mathematical knowledge). The basic idea in each of these is that we never perceive external objects directly, but only through the contents of our own mind, the images the external objects produce in us. Since sense experience never puts us in contact with the objects themselves, but only with mental images, sense perception provides no certainty that there is anything in the external world that corresponds to the images we have in our mind. Contrary to common sense, we have only our impressions of things and not direct experience of things themselves. Thus, Descartes undermines the everyday, common sense that we take for granted: that there is a world which we perceive.
In the Meditations, Descartes shows that we have good reasons to doubt all kinds of experiential knowledge. Descartes argued that, in fact, there is only one thing that cannot be doubted in this way is the existence of the mind which does the doubting.
“I have convinced myself that there is absolutely nothing in the world, no sky, no earth, no minds, no bodies. Does it now follow that I too do not exist? No: if I convinced myself of something then I certainly existed. But there is a deceiver of supreme power and cunning who is deliberately and constantly deceiving me. In that case I too undoubtedly exist, if he is deceiving me; and let him deceive me as much as he can, he will never bring it about that I am nothing so long as I think that I am something. So after considering everything very thoroughly, I must finally conclude that this proposition, I am, I exist, is necessarily true whenever it is put forward by me or conceived in my mind.”
Meditation 2, Oeuvres de Descartes, Adam, Charles, and Paul Tannery, (eds.) 1904. Paris: J. Vrin Vol. 7 p.25
This thought finds its canonical expression in the motto Cogito Ergo Sum or “I am thinking, therefore I am”(Discourse on Method). Once he has arrived at this point in his discourse, Descartes begins to rebuild his picture of the world as a dualism between mind and body. Descartes was the first to clearly identify the mind with consciousness and self-awareness and to distinguish this from the brain, which was the seat of intelligence. Hence, he was the first to formulate the mind/body problem in the form in which it still exists today. The important point for our purposes is that Descartes provides a comprehensive account of the skeptical problem re: the external world.You can find an online copy of the Meditations by Descartes at http://www.wright.edu/cola/descartes/.See http://www.wright.edu/cola/descartes/synopsis.html for Descartes’ own synopsis of the text. Tom Sorrell’s book on Descartes is a good, accessible introduction and can be found in the library.