The problem with this is, what non-believer would accept (1) unless “objective moral values and duties” were defined in such a way as to require God’s existence (making it circular)? To see this more clearly, reword (1) equivalently: “If there are objective moral values and duties, God exists” (contrapositive). Then switch the order of the premises, which of course is equivalent. The result:
Premise 1: Objective moral values and duties exist.
Premise 2: If there are objective moral values and duties, God exists.
Conclusion: Therefore God exists.
I take exception to Premise 1, for reasons I will cover below. But even if I didn’t, I see no reason to accept Premise 2, which is equivalent to saying that objective moral values and duties require the existence of God. As it stands, it looks like a bald assertion. You could define “objective moral values” so that it is not a bald assertion, but then that is where the action is (the definitions). I can only await a set of definitions that (a) are coherent, (b) make the premises acceptable, and (c) make the argument valid/non-circular (especially without using definitions that depend on Divine Commands). I don’t think all three of those can be accomplished simultaneously, but anyone is welcome to try.
Doesn’t it seem like a good idea to fully define your terms before you use them in an argument? I mean, you say the morality will be “grounded” later, but I don’t see in what sense the word morality is even defined in this context until it is so grounded.
Some people might use the subjectivism of economics as justification for complete value subjectivism in all areas, but as you say that would be mistaken reasoning. You are also correct about that big non-sequitur you mentioned. In contrast, the reason I find all value to be fundamentally subjective is that subjectivity is inherent in my interpretation of the word value itself. Why should you care how I interpret the word value? Because you just might interpret it in the same way. Stop me where we differ:
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It seems to me that the noun value is just a nominalization of the verb to value. Just as we can’t have exploitation without an exploiter, I don’t see what it would even mean for there to be value without some entity doing the valuing. (Could be man, God, or anyone.)
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Since I reject methodological collectivism (in both economics and philosophy in general), the entity doing the valuing has to be an individual, not a group. (Could still be God, any individual person, etc.)
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Therefore the only way I see to interpret the noun value is to refer to some individual entity valuing something (the act of individual valuation).
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The act of individual valuation of something is exactly what I mean by the term subjective valuation, A.K.A., subjective value. Hence, the only way I can see to interpret the noun value is as (automatically) “subjective value.” All value is subjective because all acts of valuation are subjective (that is, done by an individual, even if it is God doing the valuing).
All that said, I think it would be much clearer if we never used the word value as a noun, but instead substituted a word like valuing or valuation. That way, the connection to the verb “to value” would not be lost, and then - I think - anyone who adheres to methodological individualism would automatically understand the word valuation to refer to a subjective phenomenon (something an individual* does).
*Again, the valuer could even be God - a state of affairs which may have implications that could reasonably be called moral, though perhaps not “objective moral value.”
Also, to say that all value is subjective does not disallow that people are accountable to what God values. It also does not disallow the possibility that God may have made every human being have a certain moral compass (universal subjective valuation). Although I reject proofs of God’s existence via objective moral values, I equally reject the method of disproof of God’s existence via subjective moral values/valuation. The subjective nature of valuation has to do with how lanugage is used (and indivualism vs. collectivism, if applicable), so it can say nothing directly about the existence or non-existence of God.