 Science textbooks teem with contingent abstract entities: the equator, Jupiter’s center of gravity, the European Space Agency’s budget, etc. Twentieth century mathematics made sets central. Sets are defined in terms of their members. Therefore, any set that contains a contingent entity is itself a contingent entity. Any set that contains Cameron Winklevoss is an abstract entity that has no weight or color or electric charge. But it still depends on Cameron Winklevoss for its existence.