I'm saying that we can define what constitutes understanding by defining what constitutes reasoning within a given inference system. That's a simpler task, but

I'm saying that we can define what constitutes understanding by defining what constitutes reasoning within a given inference system. That's a simpler task, but doesn't solve the original problem. It just passes the buck. Now instead of asking whether a given learner performs reasoning and understands, we could ask whether the language/logic constitutes understanding. I'd definitely say that the current theorem provers have some form of true understanding, even though it isn't the same form as we have. For the typical ML learners, I think it's more interesting to ask and taxonomize "in what ways is this reasoning/understanding" rather than just ask "is this reasoning/understanding?"