 Essentiality can be explained by referencing necessity and/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.In other words, an object X composed of parts a, b, c and d ceases to exist if it loses part d. Additionally, X ceases to exist if it gains a new part e.

Mereological essentialism is a position defended in the debate regarding material constitution.