Mereological essentialism is a philosophical thesis about the relationship between wholes and their parts, and the conditions for their persistence. It holds the view that objects have their parts essentially, implying that if an object were to lose or gain a part, it would cease to exist—that is, it would no longer be the original object but a new, different one.
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 may also be applied to a set or proposition. A proposition, or thought, if mereological essentialism is correct, 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.
Mereological essentialism is a position defended in the debate regarding material constitution. What is the relationship between, for instance, a statue and the lump of clay from which it is made. Several different answers are proposed. Take coincidentialism, the view that there 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.
The following philosophers have thought mereological essentialism to be true; before the 20th century, Peter Abelard and Gottfried Leibniz. In the 20th century, we have G.E. Moore, Roderick Chisholm and James Van Cleve. The last two philosophers consider objects as enduring. Michael Jubien and Mark Heller, writing from the end of the 20th and the beginning of the 21st century, defend mereological essentialism for perduring objects.
There are several arguments for mereological essentialism. Some are more formal and others take mereological essentialism as solutions to philosophical puzzles or paradoxes. (This approach is, for instance, mentioned in Olson (2006).)
The argument from bad alternatives
What would be the opposite of mereological essentialism? It would be that objects would survive the loss of any part, call it mereological inessentialism. But mereological inessentialism means that a table would survive replacement or loss of any of its parts. By successive replacement we could change the parts of the table so in the end it would look like a chair. This is a version of a Sorites paradox. Because it is hard to find a middle place at which to stop, the best way could be to defend mereological essentialism (Chisholm 1973).
"Deon and Theon" argument
Imagine a person called Deon. He has a proper part, his foot. One day he loses his foot. The resulting entity is then known as Theon. But it seems that Theon existed when Deon existed, being a proper part of Deon—namely the complement of Deon's foot, Deon minus his foot. Did Deon survive? If he did, Deon and Theon are identical and Theon is a proper part of Deon. This, however, is impossible.
One way to solve this puzzle is to deny that Deon has any proper parts. Defending this view is rejecting the principle of arbitrary undetached parts (Van Inwagen 1981). It means that a cup in front of you doesn't have a left part, a right part, a part where the ear of the cup is or a part where the coffee is stored (if the hole of the cup is a part of the cup).
A world made only of stuff
Some philosophers reject the existence of individual objects. The world does not contain single, individuable objects which we can use logic to quantify. Instead the world only contains stuff or masses of matter. Masses of matter come in different quantities. We have for instance a gram of gold. There is a grammatical difference between stuff and things. We cannot say, take a gold, but must say a lump of gold (Simons 1987). Our standard way of quantifying is at most a way for the mind to project thinghood onto the world. If the world is made only of stuff, mereological essentialism must be true.
The argument from a world made only of stuff was first noted by van Cleve (1986). Defenders of a stuff ontology are Michael Jubien (1993) and Mark Heller (1990).
Because mereology is a new branch of formal systems clear arguments against mereological essentialism have not yet been raised. The most common counterargument is that mereological essentialism entails that an object which undergoes a subtle change is not the same object. This seems to be directly contrary to common sense. For example, if my car gets a flat tire and I then replace the tire, mereological essentialism entails that it is not the same car. This goes against a commonly held supposition.
The argument from a paradigmatic example
The most common argument against mereological essentialism is the view that it cannot be universally true. Take us, ourself, persons. We are humans, living organisms. As organisms we survive by having our parts replaced by, for instance, metabolism or organ transplantation. Or as humans we might have our hair or fingernails cut. All of these procedures do not seem to lead to the nonexistence of the person or for that matter the nonexistence of the living organism. Therefore mereological essentialism cannot be universally true (Plantinga 1975).
Also, if the mereological essentialist believes in presentism, then this argument may fail to convince them. A person who believes in presentism believes that the present is the only relevantly true world. This view is a response to the problem of Qualitative Change.
- Chisholm, Roderick, 1973, “Parts as Essential to Their Wholes”, Review of Metaphysics, 26:581-603.
- Chisholm, Roderick, 1975, “Mereological Essentialism: Further Considerations”, Review of Metaphysics, 28:477-84.
- Chisholm, Roderick, 1979, “Person and Object, A Metaphysical Study”. Open Court.
- Heller, Mark, 1990, The Ontology of Physical Objects: Four-dimensional hunks of matter, Cambridge: Cambridge University Press.
- Jubien, Michael, 1993, Ontology, Modality, and Fallacy of Reference, Cambridge: Cambridge University Press.
- Olson, Eric, 2006, "The Paradox of Increase", The Monist 89:3.
- Plantinga, Alvin, 1975, “On mereological essentialism”, Review of Metaphysics, 28:468-76.
- Simons, Peter, 1987. Parts: A Study in Ontology. Oxford Univ. Press.
- van Cleve, James, 1986, ”Mereological Essentialism, Mereological Conjunctivism and Identity Through Time”, i P. French, T. Uehling och H. Wettstein, eds., Midwest Studies in Philosophy, xi (Minneapolis: University of Minnesota Press.)
- van Inwagen, P., 1981, “The Doctrine of Arbitrary Undetached Parts”, Pacific Philosophical Quarterly, 62:123-37.