 One suggestion that may help to make grounding more precise would be to focus on priority. To reiterate an example used above, if the members of a set are prior to the set itself, then the existence of the set is grounded in its members. So the grounding entities are prior to—or more fundamental than—the grounded entities. Metaphors abound, but a typical way to express the idea is to say that x is fundamental or ontologically independent in this sense if and only if nothing grounds x (Schaffer 2009: 373). This is another sense in which grounding would seem to come apart from certain types of ontological dependence, as a purely modal-existential understanding of dependence in the lines of  (EDR)  (if one accepts them), without any claim to priority, is also possible. This is of course the sense which we defined above in terms of necessitation: even if x rigidly necessitates y that does not entail that y must be ontologically prior to x. At any rate, it would be odd to say that parents are more fundamental than their children, even if there is a (temporally relativized) rigid existential dependency between parents and their children.