 The mereological status of these relations, however, is controversial. For instance, although the constitution relation exemplified in (9) was included by Aristotle in his threefold taxonomy of parthood (Metaphysics, Δ, 1023b), many contemporary authors would rather construe it as a sui generis, non-mereological relation (see e.g. Wiggins 1980, Rea 1995, Baker 1997, Evnine 2011) or else as the relation of identity (Noonan 1993, Pickel 2010), possibly contingent or occasional identity (Gibbard 1975, Robinson 1982, Gallois 1998). Similarly, the ingredient-mixture relationship exemplified in (10) is of dubious mereological status, as the ingredients may undergo significant chemical transformations that alter the structural characteristics they have in isolation (Sharvy 1983, Bogen 1995, Fine 1995a, Needham 2007). As for cases such as (11), there is disagreement concerning whether teams and other groups should be regarded as genuine mereological wholes, and while there are philosophers who do think so (from Oppenheim and Putnam 1958 to Quinton 1976, Copp 1984, Martin 1988, and Sheehy 2006), many are inclined to regard groups as entities of a different sort and to construe the relation of group membership as distinct from parthood (see e.g. Simons 1980, Ruben 1983, Gilbert 1989, Meixner 1997, Uzquiano 2004, Effingham 2010b, and Ritchie 2013 for different proposals). For all these reasons, here we shall take mereology to be concerned mainly with the principles governing the relation exemplified in (1)–(8), leaving it open whether one or more such broader uses of ‘part’ may themselves be subjected to mereological treatments of some sort. 