Mereological essentialism

Mereological essentialism is a philosophical thesis about the relationship between wholes, their parts, and the conditions of their persistence. According to mereological essentialism, objects have their parts necessarily. If an object were to lose or gain a part, it would cease to exist; it would no longer be the original object but a new and different one.

Mereological essentialism requires elaboration. To what types of objects (abstract or concrete) does mereological essentialism apply? Usually, mereological essentialism is taken to be a thesis about concrete material objects, but it may also be applied to a set or proposition. If mereological essentialism is correct, a proposition, or thought, has its parts essentially; the concepts that make up the proposition are essential to it.

In the case of material concrete objects, mereological essentialism can be true in different senses depending on how objects are thought to persist through time. The two prominent, competing models are endurantism and perdurantism. It is important to note that neither endurantism nor perdurantism imply mereological essentialism. One can be an advocate for either model without being committed to accepting mereological essentialism. Within an endurantist framework, objects are extended within space; i.e., objects are collections of spatial parts. Objects persist through change (or endure) by being wholly present at every instant of time. According to mereological essentialism, enduring objects have only their spatial parts essentially. Within a perdurantist framework, objects are extended through space-time. Instead of having only spatial parts, objects have parts in both space and time. Under a framework that combines mereological essentialism and perdurantism, objects have both their temporal parts and their spatial parts essentially.

What does it mean for an object to have something essentially? 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, object X ceases to exist if it gains a new part e.

Mereological essentialism is a position defended in the debate regarding material constitution. What is the relationship between a statue and the lump of clay from which it is made? Several different answers have been proposed. Coincidentialism is the view that the statue and the lump of clay are two objects located at the same place. The lump of clay should be distinguished from the statue because they have different persistence conditions. The lump would not survive the loss of a very small part of clay, but the statue would. The statue would not survive being squashed into a ball, but the lump of clay would.

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