 Exactly how ‘ξ’ should be construed is, of course, an important question by itself—a version of what van Inwagen (1987, 1990) calls the “Special Composition Question”. A natural choice would be to identify ξ with mereological overlap, the rationale being that such a relation establishes an important tie between what may count as two distinct parts of a larger whole. As we shall see (Section 4.5), with ξ so construed (P.11ξ) is indeed rather uncontroversial. By contrast, the most liberal choice would be to identify ξ with the universal relation, in which case (P.11ξ) would reduce to its consequent and assert the existence of an upper bound for any pair of entities x and y. An axiom of this sort was used, for instance, in Whitehead's (1919, 1920) mereology of  events.[17]  In any case, and regardless of any specific choice, it is apparent that (P.11ξ) does not express a strong condition on composition, as the consequent is trivially satisfied in any domain that includes a universal entity of which everything is part, or any entity sufficiently large to include both x and y as parts regardless of how they are related.