There have been some excellent questions about whether moral claims can be objectively true or not.
Isn't there an unspoken presupposition to that argument, however?
"Moral claims can only exist in situations where there are beings who are subject to morality present in the first place." or perhaps you can word it better to capture what I am trying to say.
In other words, if there were no sentient beings, then the concept of morality could not even exist, as only sentient beings are capable of moral reflection in the first place.
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