Full Idea
Extensional mereology defs: 'distinct' things have no parts in common; a 'fusion' has some things all of which are parts, with no further parts. Axioms: (transitivity) a part of a part is part of the whole; (sums) any things have a unique fusion.
Gist of Idea
Extensional mereology needs two definitions and two axioms
Source
Keith Hossack (Plurals and Complexes [2000], 5)
Book Reference
-: 'British Soc for the Philosophy of Science' [-], p.423