 BonJour maintains that we directly grasp the necessity of  (RG).  For him, (RG) is self-evident or just seems necessarily true. So, on the view that intuitions justify, our intuitions provide at least prima facie justification for (RG). (See Levin 2007 for skepticism about the justificatory status of intuitions in relation to essentialism, such as the essentiality of origins or fundamental kind; for further discussion see  arguments for origin essentialism  in the entry on  essential vs. accidental properties.)