According to Rene Descartes, there is the reality and the dream/illusion, and however sure you may be that you are in the reality, you can never be sure that you, in fact, are not having your senses tricked and through that, or through some other means, that your body and everything you see, in fact, in the illusion.
But he ALSO states that because he has a self-awareness/soul/spirit of being, he knows that he DOES, in fact, EXIST- but in what form and where are constantly in doubt.
I am aware I exist, so I do exist, I am a seperate account- but perhaps YOU, thucydides, are an illusion.
(Let's all humour, eh?) :) Still, love Descartes, brilliant philosopher.......