 Similar considerations apply to the transitivity principle, (17). On the one hand, several authors have observed that many legitimate senses of ‘part’ are non-transitive, fostering the study of mereologies in which (17) may fail (Pietruszczak 2014). Examples would include: (i) a biological subunit of a cell is not a part of the organ(ism) of which that cell is a part; (ii) a handle can be part of a door and the door of a house, though a handle is never part of a house; (iii) my fingers are part of me and I am part of the team, yet my fingers are not part of the team. (See again Rescher 1955 along with Cruse 1979 and Winston et al. 1987, respectively; for other examples see Iris et al. 1988, Moltmann 1997, Hossack 2000, Johnston 2002, 2005, Johansson 2004, 2006, and Fiorini et al. 2014). Arguably, however, such misgivings stem again from the ambiguity of the English word ‘part’. What counts as a biological subunit of a cell may not count as a subunit, i.e., a distinguished part of the organ, but that is not to say that it is not part of the organ at all. Similarly, if there is a sense of ‘part’ in which a handle is not part of the house to which it belongs, or my fingers not part of my team, it is a restricted sense: the handle is not a functional part of the house, though it is a functional part of the door and the door a functional part of the house; my fingers are not directly part of the team, though they are directly part of me and I am directly part of the team. (Concerning this last case, Uzquiano 2004: 136–137, Schmitt 2003: 34, and Effingham 2010b: 255 actually read (iii) as a reductio of the very idea that the group-membership relation is a genuine case of parthood, as mentioned above ad (11).) It is obvious that if the interpretation of ‘part’ is narrowed by additional conditions, e.g., by requiring that parts make a functional or direct contribution to the whole, then transitivity may fail. In general, if x is a φ-part of y and y is a φ-part of z, x need not be a φ-part of z: the predicate modifier ‘φ’ may not distribute over parthood. But that shows the non-transitivity of ‘φ-part’, not of ‘part’, and within a sufficiently general framework this can easily be expressed with the help of explicit predicate modifiers (Varzi 2006a; Vieu 2006; Garbacz 2007). On the other hand, there is again a genuine worry that, regardless of any ambiguity concerning the intended interpretation of ‘part’, (17) expresses a substantive metaphysical thesis and cannot, therefore, be taken for granted. For example, it turns out that time-travel and multi-location scenarios such as those mentioned in relation to (16) may also result in violations of the transitivity of both parthood (Effingham 2010a) and proper parthood (Gilmore 2009; Kleinschmidt 2011). And the same could be said of cases that involve no such exotica. For instance, Gilmore (2014) brings attention to the popular theory of structured propositions originated with Russell (1903). Already Frege (1976: 79) pointed out that if the constituents of a proposition are construed mereologically as (proper) parts, then we have a problem: assuming that Mount Etna is literally part of the proposition that Etna is higher than Vesuvius, each individual piece of solidified lava that is part of Etna would also be part of that proposition, which is absurd. The worse for Russell's theory of structured propositions, said Frege. The worse, one could reply, for the transitivity of parthood (short of claiming that the argument involves yet another equivocation on ‘part of’).