Concerning descartes.....

I definitely think that Descartes was monumental in terms of his contributions to philosophical thought.  He questioned true existence and what we can actually know.  My only criticism would be that descartes's theory that all of our thoughts could be controlled by demons is somewhat impractical.  Now, of course no one can disprove this, but I feel like his original theory, knowledge should be determined by reason and logic, is more appropriate.  What does everyone else think?