  One report credits Democritus and Leucippus with the view that thought as well as sensation are caused by images impinging on the body from outside, and that thought as much as perception depends on images (DK 67A30).  Thought as well as perception are described as changes in the body.  Democritus apparently recognized that his view gives rise to an epistemological problem: it takes our knowledge of the world to be derived from our sense experience, but the senses themselves not to be in direct contact with the nature of things, thus leaving room for omission or error.  A famous fragment may be responding to such a skeptical line of thought by accusing the mind of overthrowing the senses, though those are its only access to the truth (DK68B125).  Other passages talk of a gap between what we can perceive and what really exists (DK 68B6–10; 117).  But the fact that atoms are not perceptible means that our knowledge of their properties is always based on analogy from the things of the visible world.  Moreover, the senses report properties that the atoms don't really possess, like colors and tastes.  Thus the potential for doubt about our knowledge of the external world looms large.  