While everything you said is true, I think it misses the point. It's clear that in order to have a reasonable discussion, we need to have two reasonable people who are willing to discuss. And you may just include "is not a conspiracy theory nutjob" or "is able to express his/her point of view in a clear way" in your definition of reasonable (of course, you
should'nt include "agrees with me" in your definition of reasonable").
The point is that you should try it out to see whether they're "reasonable". And if they are, then you're supposed to have an open, genuine discussion with them.