Is you existing equally difficult to prove as it is any of the many versions of gods?
Well, it was about the real world to start with.
But yes, even - I think, therefore I am - has its problem, as there it is no proof, that it is an "I" in any meaningful sense. In a formal sense, all it states is that something is going on, therefor something is going on, as the problem is this in all those versions:
If I can't trust any of my experience, all I know, is that I can't trust anything, except that I can doubt. But what does say about I, other than I doubt.
So if effect I doubt and that is all, I am.
And that is how we in effect get the assumptions that you are in the universe, that is real, orderly and knowable. Thus you are an "I" with experiences of the past, a life and other humans around you. And the same is the case for me.
Most people think that "I" with the experience of doubting mean they are real otherwise. But that is the exact opposite of what is real. The only thing real is, I doubt and that is all I am as real.
No one has solved that one in all its versions and that is how we get methodological naturalism.
F1fan, you have to remember I learned to be a skeptic by listening to your tribe. Don't take anything for granted and check with skepticism and critical thinking. I just didn't stop at religion.