Descartes' most famous thought. From Methodology.

Literally: "When I doubt the existence of everything, I don't have to doubt my own thoughts, because the only thing I can be sure of at this time is the existence of my own thoughts". Descartes thinks that when I doubt everything, I can't doubt the existence of the "I" who is doubting. Because this "doubt" itself is an ideological activity. And the essence of this thinking and doubting "I" is also an ideological activity. Note that "I" here does not refer to me who combines body and mind, but refers to an independent mind.

Deep meaning: Descartes' philosophical proposition uses the so-called "suspicious method" to verify whether the source of "knowledge" is reliable. We can doubt everything around us, but there is only one thing we can't doubt, and that is: to doubt the existence of the "I" who is doubting. In other words, we can't doubt our doubts, because only in this way can we affirm our doubts. Descartes also proved "the existence of God" from his "I think therefore I am". Because the subject of "I" can't be doubted, there is a higher "being" that makes "I" exist. In other words, because I exist, there must be a "being" that makes me exist, and that "being" that makes me exist must also be the "being" that makes everything exist. Therefore, the "being" who can make all things exist is bound to be possible only by God.