Descartes turned philosophy on its head and started again. First, he had to find out what he knew for certain. he did this by doubting everything. Could he believe what he saw with his eyes, when big things looked small in the distance? How did he know that he really existed? May be he was dreaming. Having to doubt his own existence almost drove  Descartes mad. Luckily, he came up with the answer: “I think, therefore I am.”

Descartes trusted maths because asleep or awake two plus three always equals five. And awake or dreaming, he was in no doubt at all that he was thinking. And if he could think, then he must exist. He decided that reality is made up of two separate things: mind and body.

But ironically, his being (not losing his life) was not because of his thinking. At the time the Church was persecuting Galileo  and other scientists for their scientific theories, Descartes was careful and moved to Holland, where they were more relaxed about new ideas and his scientific theories were tolerated. It is said that he snatched back his work from his publisher when he thought he might get into trouble with the Church. That why he would be better to say that: “I am cautious, therefore I am!”

Descartes Hated the cold and usually sat in a stove-heated room. ( A mistranslation of old French texts said he sat in a stove). He corresponded  with the queen of Sweden,who invited him to tutor her. But Sweden’s icy climate and the fact that the queen could only see him for early morning lessons did not agree with Descartes, who hated cold weather and getting up early. The combination of these two things had a bad effect on his health. A cold turned to pneumonia, and he died.