Peter Unger, "An Argument for Skepticism" – is metacertainty necessarily dogmatic?

In a previous post on certainty, I discussed Peter Unger's challenge that certainty is dogmatic. I offered a view that allows me to capture both certainty and open-mindedness. But here's a point in Unger's favor: 

 

You might argue that to assert something as being certainly true, or to assert that something certainly exists, you must also therefore assert that your analysis of what it means for something to be true is correct, or that your analysis of what it means for something to exist is correct. But you probably don't have, and shouldn't have, this further certainty, given the widespread disagreement among professionals over what it means for something to be true or to exist.

 

My solution before applies here too: by saying "I can't imagine that being false" or "I can't imagine that not existing", I am open to the possibility that it's either possible or impossible that someone could successfully imagine the truth or existence of the thing in question. If it turns out that someone can imagine the truth or existence of the thing in question, then one explanation for why they can imagine this when I can't is exactly because they have the right analysis of truth or existence when I don't.

 

But this works both ways: it may be impossible for anyone to successfully imagine the truth or existence of the thing question, and part of the explanation for this could be because the correct analysis of truth or existence entails the truth or existence of the thing in question.