To have a meaningful or maybe productive disagreement you should be able to identify what it is you disagree about. Once, for example, I had a disagreement with a neighbor over whether some or other species of vine was an invasive (it was and she was right). It was easy in that case to point to the source of our disagreement: some factual claim about Boston Ivy (irrelevant side note: there are no climbing ivy species native to Chicago). Crucially, it was also easy to point to a source for confidence in such claims about plants: a plant manual (or something like that).

Sometimes, however, it’s easy to point to what you disagree about, but not easy to find a solution–this is because you disagree about what a solution would be. This is a deep disagreement (check on this project on the topic). You disagree so fundamentally that you disagree about disagreeing.

On this topic today I learned, courtesy of Dr. Sara J.Uckelman’s Medieval Logic and Semantics blog, a Latin phrase for this situation:

Contra negantem principia non est disputandum

Or: “against someone who denies principles there can’t be a debate.”

Well, in some cases, according to Duns Scotus, there is one thing you can do:

Et ideo negantes talia manifesta indigent poena vel scientia vel sensu, quia secundum Avicennam primo Metaphysicae : Negantes primum principium sunt vapulandi vel exponendi igni, quousque concedant quod non est idem comburi et non comburi, vapulari et non vapulari.

And thus those who deny such manifest things need punishment or knowledge or sense, because, According to Avicenna (I Metaphysics): those denying a first principle ought to be beaten or burnt until they concede that being burned is not the same as not being burned and being beaten is not the same as not being beaten.

There you might have a valid case of ad baculum, though I don’t recommend this as a general principle.