During the 17th and 18th centuries, in Europe the Enlightenment introduced the scientific method, which reshaped their conception of truth. Philosophers in the Jewish, Muslim, and Christian worlds during the Middle Ages constructed metaphysical frameworks, not unlike those of Plato and Aristotle. Science was quickly showing them they could not trust their senses. Even though it looks like the sun moves around the Earth, the opposite is in fact the case. After a discovery like this, Science said that the truth is uncovered only when we insist upon proof.
Descartes was a groundbreaking physicist, mathematician, and logician. When it came time for him to direct his efforts towards philosophy, he used the logically rigorous approach with which he was already familiar. He had come to assume he held many false beliefs, and the fact that he could be dreaming or hallucinating meant that he could not correct his false beliefs through his senses.
In order to establish only what can be proven true with certainty, Descartes proposes a philosophical experiment where he doubts everything that can be reasonably doubted. This is the doubt methodology. He even doubts mathematical truths because he proposes that a demon could be deceiving him, and that 2+2 do not actually equal 4.
At this point he recognizes how difficult it is to doubt everything, and how uncomfortable it makes him feel. But, in order to remove all bias, to ensure he finds what can only be known with certainty, he proceeds.
It then occurs to Descartes that the fact that he is doubting, the fact that he is thinking, cannot be doubted. Thinking implies a thinker, an “I.”
In order to avoid being too hasty, Descartes asks what the “I” is. The “I” is not the body he normally associates with himself, nor is it a “thin vapor” that inhabits the body. It is a thinking thing that doubts, understands, affirms, denies, is willing, is unwilling, imagines, and has sensory perceptions.
He then investigates the sensory perceptions. He describes a scented piece of wax that loses its scent, shape, texture, and other qualities Although it is different, he assumes it is the same piece of wax. At this point, Descartes must focus on the fact that the wax is an experience in his “I,” his mind. His “I” is the thing in which he has the most confidence.