 This is not uncontentious. For instance, to some philosophers the thought that such abstract entities may be structured mereologically cannot be reconciled with their being universals. To adapt an example from Lewis (1986a), if the letter-type ‘m’ is part of the word-type ‘mereology’, then so is the letter-type ‘e’. But there are two occurrences of ‘e’ in ‘mereology’. Shall we say that the letter is part of the word twice over? Likewise, if carbon is part of methane, then so is hydrogen. But each methane molecule consists of one carbon atom and four hydrogen atoms. Shall we say that hydrogen is part of methane four times over? What could that possibly mean? How can one thing be part of another more than once? These are pressing questions, and the friend of structured universals may want to respond by conceding that the relevant building relation is not parthood but, rather, a non-mereological mode of composition (Armstrong 1986, 1988). However, other options are open, including some that take the difficulty at face value from a mereological standpoint (see e.g. Bigelow and Pargetter 1989, Hawley 2010, Mormann 2010, Bader 2013, and Forrest 2013, forthcoming; see also D. Smith 2009: §4, K. Bennett 2013, Fisher 2013, and Cotnoir 2013b: §4, 2015 for explicit discussion of the idea of being part-related “many times over”). Whether such options are viable may be controversial. Yet their availability bears witness to the full generality of the notion of parthood that mereology seeks to characterize. In this sense, the point to be stressed is metaphilosophical. For while Leśniewski's and Leonard and Goodman's original formulations betray a nominalistic stand, reflecting a conception of mereology as an ontologically parsimonious alternative to set theory, there is no necessary link between the analysis of parthood relations and the philosophical position of  nominalism.[5]  As a formal theory (in Husserl's sense of ‘formal’, i.e., as opposed to ‘material’) mereology is simply an attempt to lay down the general principles underlying the relationships between an entity and its constituent parts, whatever the nature of the entity, just as set theory is an attempt to lay down the principles underlying the relationships between a set and its members. Unlike set theory, mereology is not committed to the existence of abstracta: the whole can be as concrete as the parts. But mereology carries no nominalistic commitment to concreta either: the parts can be as abstract as the whole.