A fundamental mereological question is: when do some parts compose a larger whole? Unrestricted composition says anything composes something (permissive, ontologically extravagant). Restricted composition says only certain parts compose wholes. Mereological nihilism denies composition entirely. The composition principle adopted determines what objects exist and shapes the entire metaphysical landscape.
From mereology-basics, you know the Special Composition Question: under what circumstances do some things compose a further thing? Composition principles are the systematic answers, and they come with dramatic ontological consequences. The choice between them is not merely technical — it determines what kinds of objects exist at all and shapes every downstream problem in material ontology.
Unrestricted composition (mereological universalism) says that for *any* collection of objects whatsoever, there exists a mereological sum of those objects. There is an object composed of the Eiffel Tower, your left shoe, and a particular electron on Mars. This object is not natural, useful, or interesting — but by universalism's lights, it exists. The view is metaphysically maximalist and formally clean: no theory of *when* composition occurs is needed, because it always occurs. The cost is extreme ontological extravagance — the universe is populated with countless gerrymandered, arbitrary composite objects alongside familiar ones. Universalists typically bite this bullet by insisting that "exists" is cheap and ontological commitment costs less than we suppose.
Mereological nihilism takes the opposite extreme: composition *never* occurs. There are no composite objects — only partless simples. What we call "a chair" is actually many simples arranged chair-wise; the chair does not strictly exist as a further entity. This view is ontologically parsimonious — it refuses to multiply entities — but creates serious problems. Personal identity becomes precarious: there is no persisting composite you, only simples arranged person-wise. The intuition that organisms, artifacts, and social institutions are real objects requires replacement by a reconstruction in terms of simple arrangements.
Restricted composition holds that composition sometimes occurs under specifiable conditions. Van Inwagen's own influential proposal — composition occurs when and only when the activities of the simples constitute a *life* — allows organisms but not artifacts or lumps of rock. This captures the intuition that your body is a genuine composite while a pile of sand is not, but it faces pressure: why should biological integration suffice while the causal integration of a thermostat and furnace does not? Any restricted view faces the sorites problem Van Inwagen himself identified — draw the line anywhere, and there will be borderline cases that seem arbitrary. The composition principle you adopt determines what objects exist, which in turn shapes how you handle material constitution, personal identity, and persistence through change.
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