1. Are there methods for proving moral principles?
2. Are there rational methods for settling moral disputes?
3. Are there criteria that will limit the range or number of acceptable moral principles or moral systems?
4. Is there anything that we can call moral knowledge without gratuitously changing the meaning of the word 'knowledge'?

I think that if we accept ethical relativism we must answer the first question with a "no." But my point is that that doesn't tell us the answer to the other three questions. And that may be where some people go wrong. They think that if there are no proofs for principles then there are no methods for settling disputes, no way to accept and reject proposed principles, and nothing that can be called moral knowledge.