Full Idea
Chisholm has an axiom: if x is a proper part of y, then necessarily if y exists then x is part of it. If x is ever part of y, they y is necessarily such that x is part of y at any time that y exists.
Gist of Idea
If x is ever part of y, then y is necessarily such that x is part of y at any time that y exists
Source
report of Roderick Chisholm (Person and Object [1976], p.149) by Peter Simons - Parts 5.3
Book Reference
Simons,Peter: 'Parts: a Study in Ontology' [OUP 1987], p.189
A Reaction
This is Chisholm's notorious mereological essentialism, that all parts are necessary, and change of part means change of thing. However, it looks to me more like a proposal about what properties are necessary, not what are essential.