Again, the claim is not that human knowledge is impossible, only that we can’t be certain that we have it. Why not? Because there is always the logical possibility that we may be wrong, for a number of reasons (hallucinations, brain-in-the-vat, Matrix, or just plain old simple human fallibility).

It’s not a question of burden of proof. The Skeptics is not saying that we are in a simulation. Only that nobody can definitely rule that out. And the Skeptic is right.

If your brain were in a vat how could you verify that there is only one ball? With your, ahem, eyes?