Knowing is an experience. Constructing an existence, logical, or mathematical, proof is an action. We can demonstrate them. That is not to say that they are true, it is to say that they are proofs. If we have constructed proofs, we may err, but it is very hard to lie. And even if one does, err, we need not hold him accountable for his error.
Speaking truthfully, constructing a proof, and possessing the ultimate truth are very different things. I can however speak truthfully, and I can construct an existence proof, and that is the most that I can do. I can know those things even if I cannot know if I possess the truth. So what does that do for me? I doesn’t tell me anything about whether I possess the ultimate truth, but it does allow me to speak truthfully to the best of my ability – and that is all that we can ask of anyone. Because it is all that is possible for anyone.
Conversely, we must ask it of anyone who seeks to place an argument into the commons the result of which would subject others to harm.