This document discusses Rene Descartes' famous phrase "I think, therefore I am" and explores various philosophical possibilities regarding the nature of human existence and reality. The author considers ideas such as humans existing as dreams of other beings, thoughts being presupposed rather than real, and the possibility that there is no external reality and humans are simply creations of brain cells. Ultimately, the author cannot say for certain what is real, but chooses to believe in their own existence and the reality of the world around them based on their ability to think, feel emotions, and experience life in a first-person view.