Ultimately, the correct grounds for belief are those characteristics shared by mathematical propositions, a certain clarity and distinctness. But before he could rest content with these grounds, Descartes believed he had to subject them to a severe test. It seems as if there is nothing preventing this possibility. Descartes's project was to validate the use of the human faculties in gaining knowledge of the world. In particular, he wished to examine the faculties of sense and reason.
Sense presents the mind with images of sight, sound, touch, taste and smell.
Throughout the history of philosophy, from the ancient Greeks onward, there has been suspicion about the ability of the senses to deliver true information about the world. The other faculty, reason, has been valued more highly, but it ran into the conflict with faith. Descartes was most concerned with showing how reason can be relied upon. Following Plato, philosophers have distinguished two functions of reason. The most familiar is reasoning, the function of inference, were we draw conclusions from given premises.