Full Idea
Mereological essentialism says that nothing else could have been the unique entity composed of certain parts except the very thing that is composed of those parts.
Gist of Idea
Mereological essentialism says an entity must have exactly those parts
Source
Ernest Sosa (Varieties of Causation [1980], 2)
Book Reference
'Causation', ed/tr. Sosa,E. /Tooley,M. [OUP 1993], p.237
A Reaction
This sounds initially implausible. It means the ship of Theseus ceases to be that ship if you change a single nail of it. Whether we say that seems optional, but if we do, it leads to the collaps of all our normal understanding of identity.