  During the twentieth century, various aspects of Descartes' philosophy were widely invoked and perhaps just as widely misinterpreted. The  first is Descartes' skepticism. In the early twentieth  century, one response to the threat of skepticism about our knowledge of the external world was to retreat to the position  that we can only know our own sense data,  where “sense data” are equated with the supposed contents of immediate sensory experience: for vision, color patches having a shape (e.g., Russell 1914). Some authors  then treated Descartes' project in  the Meditations as that of reducing human knowledge to immediate  sense data, from which knowledge of the external world was to be constructed.