 It must be noted that this problem takes it for granted: (i) that mereological sums are unproblematic and a metaphysical default, whereas complexes are problematic and in need of an explanation; and (ii) that all kinds of entities (particulars, universal properties and relations, tropes, etc.) can indeed be summed regardless of whether such entities can exist independently of others. Similar assumptions are made in formulations that make no reference to mereological sums but rather contrast lists, groupings, or sets of entities with the unified complexes such as states of affairs or bundles of tropes.