Full Idea
I assume in this book that matter is ultimately particulate. Every material being is composed of things that have no proper parts: 'elementary particles' or 'mereological atoms' or 'metaphysical simples'.
Gist of Idea
I assume matter is particulate, made up of 'simples'
Source
Peter van Inwagen (Material Beings [1990], Pref)
Book Reference
Inwagen,Peter van: 'Material Beings' [Cornell 1995], p.5
A Reaction
It may be that modern physics doesn't support this, if 'fields' is the best term for what is fundamental. Best to treat his book as hypothetical - IF there are just simples, proceed as follows.