 There are really two questions here. One is the question: what does it mean to say that x is a constituent of a proposition? The language of constituency suggests parthood; but there’s some reason to think that x’s being a constituent of a proposition isn’t a matter of x’s being a part of that proposition. This is perhaps clearest on a Russellian view, according to which ordinary physical objects can be constituents of propositions. The problem is that a thing can be a constituent of a proposition without every part of that thing being a constituent of that proposition; a proposition with me as a constituent, it seems, need not also have every single molecule that now composes me as a constituent. But that fact is inconsistent with the idea that constituency is parthood and the plausible assumption that parthood is transitive. For discussion of this and other problems, see Gilmore (2014), Keller (2013), and Merricks (2015).