51250 Locke in his .An Essay Concerning Human Understanding .and Jacques Rohault in his .Ultimate Questions: Thinking About Philosophy .use primarily ‘reason’ as their major tool for scientific research and methodological strength. He initially expresses doubt for ‘being’ and goes further to prove the reality of ‘being’. For him, when we doubt a thing or being, we then .ipso facto, by virtue of the existence of ‘doubt’, must know that what we doubt exist as a reality. Doubt we must for Descartes. Otherwise, all apparent reality is a dream or an illusion. This is not an instance of contradiction in Descartes. Here, it is the order and tool of argument is important than any other thing. .Descartes’ critics such as Johannes Caterus, FriarMarin Mersenne, Thomas Hobbes, Antoine Arnauld, Pierre Gassendi and JesuitPierre Bourdin point out quite a few criticisms. .