Skepticism. The Skeptical School was founded by Pyrrho of Elis (about 365-275 BC) in ancient Greece. It is the theory that the real truth can never be known with certainty. One traditional form of skepticism, advocated by David Hume and Immanuel Kant, questioned the capacity of the human mind to grasp and understand reality. Another traditional form of skepticism, advanced by Hindu and Buddhist philosophers, questions the very existence of a real world outside of the human mind.
In modern times the term skepticism is used colloquially to refer to disbelief in anything non-material, such as God. Interestingly, the Bible itself gives an example of skepticism when "Doubting Thomas" would not believe that Jesus had risen until he saw Him with his own eyes and put his fingers into Jesus' wounds.