How to beat a presupp at their own game

[Superjock]

Then he will ask : How do you know you exist? In fact I just had a conversation with a theist a few minutes ago and he asked me how do I know I exist? Well I said to him that I can't NOT exist. Maybe  I should have refined what I meant to say. How do we know anything at all?

You have to start somewhere - with a base presupposition that you exist. Right? I mean, we could be living in a simulation which is what he was getting at. And he kept saying well why should I try and convince someone, an atheist, that God exists when they don't even know if they exist or the universe exists.

It's a good question, what would the point be, of trying to convince a person of anything, if they have concerns about the impossibility of knowledge?

Say, if a person sits in wide eyed wonder at how another person might know their own name, or that they exist.

I'm starting to think that you may be dealing with a person who thinks he's laying brilliant rhetorical traps for the other guy, but something else just keeps happening, instead...atheism isn't the belief that knowledge is impossible - but if your friend wants to chase that bird......

Was I wrong with what I said? That I can't NOT exist? If a theist asks you how do you know you exist...