Full Idea
It is a principle of mereology that no two things consist of exactly the same atoms.
Gist of Idea
In mereology no two things consist of the same atoms
Source
David Lewis (Parts of Classes [1991], 2.3)
Book Reference
Lewis,David: 'Parts of Classes' [Blackwell 1991], p.39
A Reaction
The problem with this is screamingly obvious - that the same atoms might differ in structure. Lewis did refer to this problem, but seems to try to wriggle out of it, in Idea 15444.
Related Ideas
Idea 15444 Different things (a toy house and toy car) can be made of the same parts at different times [Lewis]
Idea 15445 Butane and Isobutane have the same atoms, but different structures [Lewis]