Most upvoted comment
Top rated philosophy books on Reddit rank no. 37
Ethics and truth(r/askphilosophy)
Your friend’s argument is not unknown in metaethics. A popular argument for the subjectivity of moral truth is that moral knowledge comes easy if the individual or one’s society determines what is morally right and wrong; all one needs to do is examine what oneself (or one’s culture) believes about what is right and wrong, and these are things that are easily accessible, epistemically speaking.
However, objective moral properties would be another thing altogether. (Your friend talks about “absolute moral truth”, which I take him to mean “objective moral properties.”) The most popular argument of the sort you friend mentioned is one version of Mackie’s Argument from Queerness. (source: Mackie, J.L. 1977. Ethics: Inventing Right and Wrong. Harmondsworth: Penguin.) Mackie states that if there are objective moral properties, then they would be utterly odd and weird properties that would require an utterly odd and weird cognitive faculty to detect them. Perception, memory, rational intuition, and the like would all fail to detect these queer properties. Since we shouldn’t postulate such a weird cognitive faculty (like an ethical intuition), we should be skeptical about these moral properties. So even if there are these objective moral properties, we would not know of them.
Two responses to Mackie: (1) The most common contemporary response to Mackie is the ethical naturalist’s. Naturalists believe that in some way moral properties are reducible to or even identical to natural properties. Since natural properties are not odd properties that require odd cognitive faculties to detect them, we can come to know what is right and wrong by knowledge about natural properties, which we come to know via our quotidian cognitive faculties. Thus, objective moral properties are knowable. (2) A response that is coming back into vogue is from the ethical non-naturalists, which are those that think that the moral realm of properties is wholly non-natural. The non-naturalist claims that Mackie’s argument begs the question. We should not be all that surprised by the existence of an ethical intuition because moral properties are easily knowable. What do I know more certainly than that it is morally wrong that torture small children for my own enjoyment? I certainly know that claim more certainly than any of Mackie’s premises in his argument. So postulating such a faculty isn’t all that detrimental.