> There isn't universal agreement about the nature of that 'you', nor of whether 'you's are thinking.

That's beside the point. We can (what that means is "should if we want to be logical") agree that "if you're thinking, you're existing" in one form or another. Even if that "you" is only some kind of deluded colony mind just west of Antares. Some "you" exists, no matter what.

Of course, since there's always someone who doesn't want to be, or can't be, logical, not even that statement is something "we all agree on."

It's just something that's undeniably true by any sane standard.

Mathematical statements are only slightly less undeniably true (or false). Some day, perhaps, though it's hard to imagine how, 2 + 2 might not equal 4. But even if that were to happen, and you were to notice it, that would prove you still existed.