All you truly know is that right now you are having a thought. Which means some entity most exist that has that thought. Everything else could be a product of your imagination, or something the beings that put you into the matrix want you to think
Or as another possibly-previously-existing possibly-conscious entity put it succinctly: I think, therefore I am
nah. there's a lag time in my imagination filling in deeper details (roughly: the more time I spend imagining an apple, the more detailed it gets) that isn't present when interrogating reality. Reality is immediately as detailed as I can examine.
And you can’t even be sure about yourself, either. All your memories could be implanted, for all you know. This could be your first conscious thought, this instant. Welcome to the Universe.
Or as another possibly-previously-existing possibly-conscious entity put it succinctly: I think, therefore I am