So how do you determine if a metaphysical argument is valid? Does each person decide for themselves if they like the argument or not?
I imagine it would be as one judges any line of reasoning: if the premises are true and the logic rigorous, the argument is valid and the conclusion sound. Any imperfection or outright failure if premises or logic will diminish our certitude as to the conclusion. But perhaps what you said earlier would make this clearer…
How do you tell if a metaphysical chain of reasoning is valid?
For mathematics, if a proof obeys the rules of logic, then you can rely on its conclusion. That doesn’t seem to be the case in metaphysics. The terms used have no precise definition and the axioms used seem completely arbitrary. So a metaphysical argument doesn’t seem to really establish anything, nor is the conclusion something that can be independently examined and analyzed.
It is probably worth distinguishing the
rigor (if I may call it that) of a chain of reasoning from its
validity. Any proof which obeys the rules of logic (i.e. in which the conclusion and premises are or can be arranged according to proper syllogistic form without equivocation) may be called rigorous, in the sense that the conclusion necessarily follows from the premises, but that does not mean it is valid. If the premises are not also true, the conclusion may be false and should not be relied on.
For instance: Every even number is divisible by two. Every prime number is even. Therefore every prime number is divisible by two. The logic is flawless, it is just that one of the premises is false, so in this case the conclusion is false too.
I don’t see why, in theory, metaphysics can’t have formal rigor in its proofs. If you are saying that, in practice, metaphysicians generally aren’t very rigorous, that is one thing, and probably true often enough. But if you are saying that metaphysics, because it is metaphysics (i.e. the study of what is most universal – and by most universal I mean most common or abstract, like ‘being’), cannot make use of rigorous logic, then I fear I fail to see why

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Concerning the validity of the premises, you raise the difficulty that, in metaphysics, the terms have no precise meaning. It is true that unlike mathematics, there are terms in metaphysics which cannot be defined according to what they are, using a “genus and species-making difference”. For instance, if I recall correctly, St. Thomas defines “essence” as “that which the account [or definition] signifies” – which, while it distinguishes “essence” from every other concept, does not tell us what essence is itself except through a sign. In addition, mathematical terms are sometimes fairly concrete and particular (though of course not always, e.g. ‘tensor’), whereas metaphysical terms are highly abstract (e.g. ‘being’ or ‘actuality’), and one might argue that ‘abstract’ is simply a fancy way of saying ‘vague’.
Still, I don’t see how either of these difficulties are, in theory, problematic for metaphysics (though of course in practice they mean that one is likely to slip up). The fact that we cannot define metaphysical terms according to genus and species does not make the definition any less precise and determinate. We can still clearly delimit our meaning such that we include all we intend and nothing we do not intend.
And again, in metaphysics as in math, the abstraction of the terms does not negate the precise use of those terms
as abstract. E.g. if I wish to write an abstract proof of the fundamental theorem of calculus which applies to both geometrical curves and algebraic functions, I cannot use any properties, axioms or theorems peculiar to the curves without first proving the algebraic analogue (e.g. the intermediate-value theorem, the geometrical equivalent of which is fairly obvious, if not axiomatic). Again, if I want to write a geometrical proof in the style of Euclid by which a perpendicular line can be dropped to an n-dimensional volume from a point not contained by the volume, I must avoid any assumptions only applicable to 3- or 4-dimensional space. Presumably there is nothing wrong if metaphysics, too, deals with abstract concepts precisely.
Let me know if I said anything false. I’m also a little unsure if what I said actually addresses your points, so let me know

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