Full Idea
Van Ingwagen writes of 'mereological nihilism' (that only mereological atoms exist) and of 'mereological universalism' (adhering to the principle of Unrestricted Composition).
Gist of Idea
Mereology is 'nihilistic' (just atoms) or 'universal' (no restrictions on what is 'whole')
Source
report of Peter van Inwagen (Material Beings [1990], p.72-) by Achille Varzi - Mereology 4.3
Book Reference
'Stanford Online Encyclopaedia of Philosophy', ed/tr. Stanford University [plato.stanford.edu], p.18
A Reaction
They both look mereologically nihilistic to me, in comparison with an account that builds on 'natural' wholes and their parts. You can only be 'unrestricted' if you view the 'wholes' in your vast ontology as pretty meaningless (as Lewis does, Idea 10660).
Related Idea
Idea 10660 A commitment to cat-fusions is not a further commitment; it is them and they are it [Lewis]