> For example, when discussing if the Earth is flat or round, you might both find agreement in that nobody has ever fallen off the edge of the Earth.

So I'm not trying to argue that the earth is flat, but how do you respond if they ask for proof that nobody has fallen off the edge of the earth?

It's not like you can point to some record of all history and show that the event doesn't exist within it. You might try to explain that there is no edge to fall off of, but if you can convince them of that, then the conversation never needed to happen in the first place.