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.
True: only sentient beings can think about moral questions, and so moral questions don't arise in a world with no sentient (or better, sapient) beings. Of course, in one sense of "arise," no questions arise unless there are creatures who can ponder the questions. Nonetheless, that doesn't make the way things are depend on the existence of thinking beings. There were electrons before we came on the scene, and there would be electrons even if neither we nor any creatures like us had existed. That said, you're right: moral matters have an intrinsic connection with beings who can ponder them. There are no live moral issues in a lifeless world, nor even in one with sentient but no sapient creatures. Moral truths are truths about how certain kinds of creature should behave if there were any. But this is consistent with there being moral truths even if nothing in the world knows those truths and even if none of the relevant kinds of creatures exist. Thus, one might say (I would) that before any thinking...