This lecture discusses the 16th century philosopher Rene Descartes' work The Discourse on Method, and focuses on his discussion in part 4 of what can be called into doubt and what cannot.

Descartes argues that any ideas that come to us through the medium of the senses can be called into doubt. Going even further, even logical and geometrical/mathematical demonstrations can be doubted. And we cannot even be sure at any given time whether we are awake or dreaming.

What cannot be doubted - for the person who is engaged in doubt - is the very fact that one exists. Here we arrive at the famous Cartesian "cogito ergo sum", or in the French of the Discourse, "je pense, donc je suis" (I think, therefore I exist).

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