 The priority argument says that grounding is a justified posit because of its direction-fixing role—it fixes the direction of priority between facts, where one fact is prior to another only if the former is more fundamental than the latter. (See  §4.3  for further discussion of fundamentality; J. Wilson (2014) initially proposes the priority argument but rejects it—more on this below.) Let a priority case be one in which some fact is prior to another and entities these facts concern are yoked by metaphysical relations. One way to cast the priority argument is as follows: there are priority cases in which the direction of metaphysical relations doesn’t determine which fact is prior to the other; and the best candidate for that which fixes the direction of priority in such cases is grounding.