Start with the Aristotelian thought that abstract entities are grounded in concrete ones. Add this principle:
- If x is grounded only in the ys, then it is impossible for x exist without at least some of the ys existing.
- x is a necessarily existing abstract entity.
- There is a possible world in which none of the actual world's contingent concrete entities exist.
If the concrete entities that ground x are all contingent, then we have a violation of the conjunction of (1)-(3), since then all the actual grounders of x could fail to exist and yet x is necessary. So:
- There is at least one necessary contingent entity among the entities grounding x.