English training Descartes, a French philosopher, tried to doubt the various things in this world. For example, an apple there may be a hallucination, and the one I am studying here may actually be a dream, and the back of a person who is facing me now may not exist. Then Descartes noticed most things, including myself of this world can doubt. So, Descartes thought that are there certainly existence which cannot be doubted? And he summarized that, and in 1637 issued the book "Discourse on the Method".