Certainty in Descartes' Meditations on First Philosophy
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?
The First Meditation of Meditations on First Philosophy, subtitled "What can be called into doubt," opens with the Meditator reflecting on the number of falsehoods he has believed during his life and on the subsequent ability of the body to deceive him. Seated alone by the fire, he resolves to demolish former opinions and rebuild his knowledge on more certain grounds.
The Meditator reasons that he need only find some reason to doubt his present opinions in order to prompt him to seek sturdier foundations for his knowledge. Rather than doubt every one of his opinions individually, he reasons that he might cast them all into doubt if he can doubt the foundations and basic principles upon which his opinions are founded.
Everything that the Meditator has accepted as most true he has come to learn from or through his senses. However, the Meditator realizes that he is often convinced when he is dreaming that he is sensing real objects. He feels certain that he is awake and sitting by the fire, but reflects that often he has dreamed this very sort of thing and been thoroughly convinced by it. On further reflection, he realizes that even simple things can be doubted. Omnipotent God could make even our conception of mathematics false. One might argue that God is supremely good and would not lead Descartes to believe falsely all these things. He supposes that not God, but some "evil demon" has committed itself to deceiving him so that everything...