Determinism is the theory that everything is caused by antecedent conditions, and such things cannot be other than how they are. Though no theory concerning this issue has been entirely successful, many theories present alternatives as to how it can be approached. Two of the most basic metaphysical theories concerning freedom and determinism are soft determinism and hard determinism.
René Descartes was the first philosopher to raise the question of how we can claim to know anything about the world with certainty. The idea is not that these doubts are probable, but that their possibility can never be entirely ruled out. If we can never be certain, how can we claim to know anything?