Definition
The above statement of mereological essentialism requires some elaboration. First, what types of objects: abstract or concrete objects? Mereological essentialism is most commonly taken to be a thesis about concrete material objects, but it can also be considered true of a set or proposition. A proposition, or thought, if mereological essentialism is true, has its parts essentially; the concepts that make up the proposition are essential to it.
Further, in the case of material concrete objects, mereological essentialism can be true in different senses depending on how such objects are thought to persist through time, these senses going under the names endurantism and perdurantism. Mereological essentialism for enduring objects - objects that persist by being wholly present every instant, means that the enduring objects only have their spatial parts essentially. Mereological essentialism for perduring objects - objects that are spread out with parts both in space and time, have also their temporal parts essentially in addition to their spatial parts.
Finally, what does it mean for an object to have something essentially? The usual way to explain essentiality is by reference to necessity or possible worlds. Mereological essentialism is then the thesis that objects have their parts necessarily or objects have their parts in every possible world in which the object exists.
Read more about this topic: Mereological Essentialism
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