Full Idea
A thing is a material object if it occupies space and endures through time and can move about in space (literally move, unlike a shadow or wave or reflection) and has a surface and has a mass and is made of a certain stuff or stuffs.
Gist of Idea
Material objects are in space and time, move, have a surface and mass, and are made of some stuff
Source
Peter van Inwagen (Material Beings [1990], 01)
Book Reference
Inwagen,Peter van: 'Material Beings' [Cornell 1995], p.17
A Reaction
It is not at all clear what electrons (which must count for him as 'simples') are made of.
Related Idea
Idea 14937 That the universe must be 'made of' something is just obsolete physics [Ladyman/Ross]