Full Idea
Mereological essentialism (whose leading advocate is Chisholm) says that for every x and y, if x is ever part of y, then y is necessarily such that x is part of y at any time that y exists.
Gist of Idea
Mereological essentialism says that every part that ensures the existence is essential
Source
Baruch Brody (Identity and Essence [1980], 5.6)
Book Reference
Brody,Baruch: 'Identity and Essence' [Princeton 1980], p.128
A Reaction
This sounds implausible, especially given the transitivity of parthood. Not only are the planks that constitute Theseus's Ship now essential to it, but all the parts of the planks, every last chip, are as well.