A response I’m getting a lot to my defence of metaphysics is that metaphysical terms or categories are “nonsense” or “meaningless”. There are two forms of this critique: (1) the stronger verificationist thesis and (2) a weaker semantic scepticism.

(1) The verificationist approach is that a claim is only meaningful or sensible if it is analytically or empirically testable/confirmable, i.e., if it is confirmable or connected to observation. The meaningfulness or sense of a claim is judged by empirical conditions. 

Classically, the flaw in verificationism is that the claim that “what is meaningful is analytically or empirically testable” is not itself analytically or empirically testable. Hence, by its own criteria, this claim would be a philosophical statement about metaphysics and thus meaningless. This puts the verificationist on the horns of a dilemma:

Either the verificationists’ claim is meaningful (in which case non-empirically testable claims can be meaningful), or it isn’t (therefore the dismissal of metaphysics as nonsense lacks any justification/is an arbitrary expression of preference). 

Either way, the verificationists’ claims are self-refuting (smuggling in philosophy while denying philosophy) – verificationism can certainly work as a methodological constraint but fails as a semantic criterion for sense. 

(2) This doesn’t necessarily touch on weaker semantic claims about the use cases or referents of metaphysical terms, though – if not empirical conditions, what renders some metaphysical terms or claims sensible and worth taking seriously from others that are not?

There are many answers to this question, from coherence, intelligibility, truth-aptness, the role the claim plays in inference, pragmatic usefulness, but my own justification would be ‘explanatory power’. Metaphysical concepts like ‘being’, ‘form’, etc. explain things even if not directly testable in a lab. Metaphysical claims and terms, while not empirically testable (always), are still accountable to reason and rationality.

In short, not every metaphysical claim can be dismissed as ‘nonsense’ or 'meaningless', but ridiculous metaphysical claims and terms can be identified by their lack of explanatory power, incoherence, lack of intelligibility, and overall, a lack of accountability to reason or rationality.