Under what circumstances do two objects compose a third? Mereological universalists answer that they do so under all circumstances, for, according to them, mereological composition is unrestricted (as it turns out, what exactly this means will be crucial here, so more on this later).

Einar Bohn (2009) has recently put forward an argument against the necessity of mereological universalism from the possibility of junky worlds – i.e. worlds in which everything is a proper part of something else (and, in which, something exists).

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The argument, which I will call the junk argument, goes as follows: Although it may be tempting for mereological universalists to think that the best strategy to defuse the junk argument is to deny (2) (see Watson 2010), I will argue...