But these people I was debating refuse to accept that there is such thing as a 'right' by that definition. They say that 'what actually happened' is forever unknowable. I was trying to point out that while we may or may not find out exactly what happened, we can always tell if an explanation is 'more right' or 'less right' than another, based on how useful it is in explaining the evidence.
It's worth asking whether your interlocutors were physical anti-realists or moral anti-realists who got quite confused about the scope of their anti-realist position.
I wrote the following on my blog last night. I thought that I'd run it past an intelligent audience. Note that what I have referred to as an idea is what we here at lesswrong would call a 'belief'. I changed the name to remove any strange foggy baggage that might appear in the heads of potential readers who are not familier with belief vs belief-in-belief and other concepts like that.
What are your thoughts?