In his landmark works Discourse on the Method (1637) and Meditations on First Philosophy (1641), René Descartes tackled a simple yet imposing question: how can one know anything for certain? Laid out in methodical detail, his answers would provide the foundation for modern philosophy and science. In this video from 1987, the celebrated UK broadcaster and philosophy populariser Bryan Magee (1930-2019) dissects Descartes’s world-changing writings alongside the UK philosopher and Descartes scholar Bernard Williams (1929-2003). In doing so, the pair touches on how the existence of God was fundamental to Descartes’s construction of the Universe, what precisely he meant in proclaiming ‘I think, therefore I am’, and which of his ideas have fallen out of fashion in contemporary philosophy.