Rene Descartes, perhaps the first great modern philosopher, discovered that much of what he was taught by his Jesuit teachers was doubtful. So troubled by the fact that "there was no such learning in the world as Ihad been led to hope," he set out to find the foundations upon which geniune, indubitable knowledge could be built. In Meditations on First Philosophy, he employed a technique of radical doubt, with the aim of identifying at least one belief he wouldn't be able to doubt.

His method was to examine each one of his beliefs, and then to abandon any of them that it was possible to doubt. In this way, he showed that it is easy enough to doubt the truth of all of our sensory experiences - we might be dreaming, and yet not be aware of it; and, most disconcertingly, that it is possible that we have been deceived about absolutely everything, even the simplest truths of mathematics, by an evil demon.

Happily, this technique also establishes that in the very act of doubting we show there must be an "I" which is doing the doubting.

As Descartes put it, "Cogito ergo sum" ("I am thinking, therefore, I exist").


Rene was sure he existed - but he wasn't sure about those other two.

What are your thoughts about Rene Descartes solutions?