Full Idea
The underlying logic of classical extensional mereology does not have the resources to deal with temporal and modal notions such as temporary part, temporal part, essential part, or essential permanent part.
Gist of Idea
Classical mereology doesn't handle temporal or modal notions very well
Source
Peter Simons (Parts [1987], Intro)
Book Reference
Simons,Peter: 'Parts: a Study in Ontology' [OUP 1987], p.1
A Reaction
Simons tries to rectify this in the later chapters of his book, with modifications rather than extensions. Since everyone struggles with temporal and modal issues of identity, we shouldn't judge too harshly.