 Premise (2)  of the argument is taken as given by those who advance this argument, but of course some would deny it.  Premise (1)  might seem strange, since we have substantive knowledge of many kinds of facts that surely are mind-independent, such as facts about the stars. But (1) is intended as a claim specifically about knowledge of normative facts. The claim is that there is something about the normative such that we could not have knowledge of facts of this kind if they were mind-independent. The claim has been supported in a variety of ways. Some have followed Benacerraf and Field’s formulation of a problem for mathematical Platonism (Benacerraf 1973 and Field 1989). Enoch (2011: ch. 7) and Setiya (2012: ch. 2) both take this to be a main epistemological worry for realism. The argument is roughly that if normative facts were mind-independent, there would be no explanation of the reliability of our substantive normative beliefs – which would undermine our substantive normative knowledge. Some think that there could be no such explanation because normative facts do not (and cannot) cause anything. It is controversial, however, whether normative facts ever have causal impact on anything. (See sec 3.2 of entry on  moral skepticism  and the extensive literature on moral explanations, including Harman 1977 and Sturgeon 1985.)