...consider the proposition that all triangles have three sides. Suppose Descartes sees that the concept triangle is the same as the concept closed, three sided, plane figure. Then he can see that the proposition simply must be true. In light of this, Descartes determines that whatever is clearly and distinctly perceived in this way, is true.

When Descartes speaks of "clear and distinct perception," he is not referring to any sort of sense perception. Rather he is referring to a special way of grasping concepts and propositions. If you contemplate a concept, such as the concept triangle, and you "see" all the simple concepts out of which it is composed, and you "see" how they are put together to make the concept triangle, then you are clearly and distinctly perceiving the concept. If you contemplate a proposition, and you clearly and distinctly perceive every concept involved in that proposition, and you "see" how they are involved in that proposition, and you "see" that they are so related that the proposition must be true, then you are clearly and distinctly perceiving the proposition. Obviously, this is not a matter of sense perception. In Descartes' view, sense perception does not yield metaphysical certainty.

(From Hobbes to Hume: A History of Western Philosophy, Vol III, by W. T. Jones. HBJ, 1969, p. 129)