@karbaasi
That's kind of a misrepresentation of Descartes's famous philosophy. It's about how you can't be certain of anything... everything COULD be an illusion or a dream. All you know is that YOU exist in some form or capacity, because someone must be thinking these thoughts.
So those people who don't think? May not exist. They PROBABLY do, but you can't know for sure.
I know, I'm ruining the joke. ^_^ But you can't know for sure that I exist either. Maybe you're just dreaming this post.