Descartes did not know what he knew.
“I realized that it was necessary... to demolish everything completely and start again right from the foundations if I wanted to establish anything at all in the sciences that was stable and likely to last.”
I will refuse to assent to what can be doubted. This includes beliefs from the senses.
Why can these beliefs be doubted?
There may be no way to know whether I am asleep or awake and, so, whether one’s experiences can be trusted.
A powerful genius may be systematically deceiving me...
He needed something that could not be doubted upon which he could ground additional beliefs and so further knowledge.
If I am deceived, then I exist.
From this foundation, he began to infer other things.
He began to draw conclusions about himself that led to what is called 'mind-body' dualism, the view that the mind and body are distinct substances.
I can know that I exist as a thinking thing while doubting that I have a body.
Doubting, believing, etc. do not imply having a body, and so are not properties of a bodily thing, and so I am not identical with a body.
The idea is that you can tell what kind of thing something is by examining its properties. The properties of a thing tell you what sort of thing it is. This is the point of the example of the piece of wax. The properties of the wax change when heat is applied to it, but they are always those of a bodily thing.
How did Descartes escape from his skepticism?
He argued for the existence of God, a perfect being. He reasoned that such a being would not permit his creatures to be systematically deceived.
His arguments were a priori. This means that they were not based on experience.
