The next “The Great Philosophers” programme is on Descartes.  Bryan Magee interviews Bernard Williams.  Descartes' “Cogito ergo sum” addresses fundamental epistemology - how can we know anything?  It was Descartes who proposed that the mind and the body are two distinct substances, starting a long-running debate.  There is an “In our time” episode, and Stanford Encyclopedia entries on "Descartes’ Life and Works” and the “Cogito”, and a longer entry than most in “The Yellow Book”.