Cogito, ergo sum

Cogito ergo sum is a Latin sentence that means "I think, therefore I am". It was first said by a famous philosopher named René Descartes.

So, let's pretend that you are sitting on a chair and wondering if you are actually there or if everything around you is just an illusion. Descartes was also wondering the same thing, but he came up with a clever idea.

He realized that even if everything around him was an illusion, he was still thinking about it. He thought, "if I am thinking, then I must exist." This is what cogito ergo sum means.

Basically, Descartes was saying that the fact that he was able to think about his own existence proves he exists. It's like saying, "if I can ask myself if I'm real, then I must be real because I'm the one doing the asking."

So, even if everything around you is an illusion, and you aren't sure if you really exist, the fact that you can think about it means you definitely do exist.