RUTH BARCAN MARCUS AND MODALITY (3): essentialism

Marcus talks about something called essentialism, and asks: when we say of something that it could have been different, might not have existed, might not have had certain properties, are we saying something real about that thing, or are we just playing with words in logic? In other words: does every object have properties that it must have in every possible situation? And if so, what does that mean for logic and for how we talk about possibility or necessity.

First, recall what she meant earlier by using a “modal logic with quantifiers.” That logic allows us to talk not only about “possibly this thing” or “necessarily that thing,” but also to talk about “some object x” or “for every object x.” That is: we can quantify over objects when we use modal notions (possible, necessary). Now she asks: suppose you pick out an object, say “Socrates” or “this apple.” And now you consider what could have been different about that object. Maybe the apple could have been green instead of red. Maybe Socrates could have existed in Worksop. Could the apple, or Socrates, have been totally different, or have had different properties, or even fail to exist? Essentialism is the view that denies that arbitrary change. According to essentialism, an object has some properties that it could not lose and still remain the same object. Those properties are its “essences.”

In logic, essentialism has big consequences. If you allow modal logic plus quantification plus identity (the notion that “x is the same as y”), then you have to think: if x = y (x is the same object as y), is that identity always true in every possible situation? Or could x and y have been different in some possible world? If you think identity is something that could vary, then you allow the possibility that “this apple might not have been this apple.” That seems very strange. Most people think that if two names refer to the same object, that object is the same in every possible world, you can’t have it be “apple #1” in one world and “apple #2” in another and still say it is the same object.

Marcus argues strongly for “the necessity of identity.” That means: if x is y, then necessarily (in every possible scenario) x is y. There is no possible world in which x and y are different, because they are the same object. Why is that important? Because if identity is not necessary, then we get all sorts of weird results. If x could have been different, then what counts as “the same object” becomes unclear. Logic becomes unreliable when we try to talk about what objects are possible, or what could have existed, or what properties they could have had. But accepting necessity of identity is not enough. We also need to think about other properties of objects. Which properties are essential? Which are accidental (could have been different)? 

Suppose you have your favourite toy car. It is red, made of plastic, and has four wheels. Could the same toy car have been blue instead? Could it have had three wheels? Could it have been made of wood instead of plastic? Essentialism says some of those questions might make sense, but only certain features are essential. Maybe “being that toy car” and “being made of material that is this car’s matter” are essential; but “its colour is red” might not be essential.

In modal logic with quantifiers, you might try to represent essential properties by saying: “For every object x, if x is this toy car, then necessarily (if x exists) x has property P.” That is: “If x is ToyCar123, then necessarily, if x exists, x is red.” If you accept essentialism, you might only allow some P as essential, not all properties. Marcus explores what happens when you try to formalise this in logic. She shows that it's possible to build a logical system that treats identity as necessary and still handles quantifiers and modality in a clean way. That system avoids some of the confusions critics like Willard Van Orman Quine raised long ago. Quine thought that modal logic made a mistake when it allowed talk about possibilities in a way that might treat sentences as objects, or treat possible but non-actual things as real “possibilities.” Marcus’s system tries to keep the logic safe: it only quantifies over actual, or at least referable, objects. 

At the same time, she warns: essentialism can be misused. If you say “Everything that exists necessarily exists,” or “All properties of an object are essential,” you end up with a very rigid world: nothing could ever change. That seems unrealistic. Objects in our world change all the time: people get haircuts, toys get painted, trees lose leaves, and so on. So a good logic must allow that many properties are accidental, not essential. Furthermore, she argues that some popular alternatives are worse. Some philosophers tried to avoid essentialism by rejecting modal logic altogether, or by weakening identity so that objects might not be the same across possible worlds. That, she says, leads to confusion: you end up treating descriptions rather than objects, or you end up with a world full of abstract “possible objects” whose connection to real things is unclear. She calls attention to the cost: you lose the ability to say meaningful things about actual objects’ possibilities. Her system preserves both identity and meaningful talk about possibility and necessity. 

In short, she wants us to think carefully about what it means to say something “could have been different.” It asks: if we pick out a thing now, a person, an apple, a car, do we really think there is a real version of that same thing in some other possible world? Or do we just imagine a different object that looks similar but isn’t the same? If identity is necessary, then the “same thing in another world” must literally be the same thing. That keeps logic safe from crazy conclusions like “the Eiffel Tower could have been a giraffe,” because that would be a different object, not the Eiffel Tower.

At a deeper level, Marcus’s exploration of essentialism in modal logic highlights a tension: we want logic to be flexible enough to talk about possibility and change, but also fixed enough to preserve identity and avoid turning everything into vague descriptions or unreal “possible things.” Her work tries to strike that balance.