The ability to think and reason implies that there is a thinker (you). Thoughts can't exist without a person to think them. Illusions can't exist without someone creating them. If you aren't actively creating them, then that means they are either created by someone else (God, in the case that only you exist) or that that person is real instead of an illusion. That was the argument that Rene Descartes came up with to prove his own existence and that he was real. Obviously it does not apply to just one person but everyone, meaning that we all do exist.
He was a French Philosopher many years ago; that's not exactly how he put it but that's the gist of it, you can read his meditations if you're interested in stuff like this. It's something I learned in philosophy this semester Lol
Edit: Just saw BeHaku's post, Descartes is already more known around here than I thought he'd be