ABSTRACT
When questions are raised about our knowledge of the .external world, two things are generally presupposed: a conception of knowledge and a conception (however vague and schematic) of "the" external world. Philosophers who have debated the questions have been in general agreement about the latter conception: the external world is the system of objects external to human consciousness that is thought to be responsible for (among other things) the sensory experiences human beings commonly have. But similar agreement does not exist about the relevant conception of knowledge. When Descartes poke of knowledge, he meant rational certainty of a special kind. His conception differs, as we shall see, from the conceptions current today, but his approach to our knowledge of the external world set a precedent hat is still admired and even emulated. It also created a nest of problems that are very difficult to resolve.
