... proof of God. Descartes begins by summarizing what he is certain he knows. His basis for this at this stage is anything he can perceive "clearly and distinctly" can be believed. He knows he is a thinking thing, and even if what he senses outside himself is not real, the sensations they cause - the thoughts or ideas - are enough to prove this. However, if the things Descartes knows clearly and distinctly, such as 2+3=5, turned out in fact to be false, he would be left with nothing. As before, there were things he believed, "the earth, the sky, the stars..." but he has learned to be dubious about this. The same could apply here, God could be deceiving him. To Descartes knowledge, God is all-powerful and could make us perceive incorrectly if He chose to. But Descartes can see no reason for this, and seeing as he has not ...

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