Full Idea
It may be metaphysically necessary in one sense that sets or universals or mereological aggregates exist, while in another sense existence is always a contingent matter.
Gist of Idea
Sets, universals and aggregates may be metaphysically necessary in one sense, but not another
Source
Gideon Rosen (The Limits of Contingency [2006], 10)
Book Reference
'Identity and Modality', ed/tr. MacBride,Fraser [OUP 2006], p.38
A Reaction
This idea depends on Idea 18856 and 18857. Personally I only think mereological aggregates and sets exist when people decide that they exist, so I don't see how they could ever be necessary. I'm unconvinced about his two concepts.
Related Ideas
Idea 18856 Non-Standard Metaphysical Necessity: when ¬P is incompatible with the nature of things [Rosen]
Idea 18857 Standard Metaphysical Necessity: P holds wherever the actual form of the world holds [Rosen]