 You might think that the above account of relative fundamentality fails to identify a necessary condition, especially if you assume that, for any two facts, one of them is more fundamental than the other or they’re equally fundamental. For example, perhaps [There are hydrogen atoms in Phoenix] is more fundamental than [There are H2O molecules in Ithaca] even though the former doesn’t ground the latter (Bennett 2017: Ch. 6). (For more sophisticated characterizations of relative fundamentality that appeal to grounding, see Bennett 2017: Ch. 6 and Werner 2021.)