Full Idea
In military usage, persons can be parts of small units, and small units parts of large ones; but persons are never parts of large units.
Gist of Idea
In the military, persons are parts of parts of large units, but not parts of those large units
Source
Nicholas Rescher (Axioms for the Part Relation [1955]), quoted by Achille Varzi - Mereology 2.1
Book Reference
'Stanford Online Encyclopaedia of Philosophy', ed/tr. Stanford University [plato.stanford.edu], p.4
A Reaction
This much-cited objection to the transitivity of the 'part' relation seems very odd. There could hardly be an army or a regiment if there weren't soldiers to make up parts of it.