Modal Realism vs. Actualism: The Metaphysics of Possible Worlds

1. The Nature of Possibility and Necessity

Before diving into the competing theories, let's briefly revisit the problem they aim to solve. Our language and thought are saturated with modal concepts. We say things like:

• "It's possible for you to have majored in chemistry instead of physics."
• "It's necessary that 2+2=4."
• "If the fine-structure constant were different, life might not have developed."

A central goal of analytic metaphysics is to provide a systematic account of what makes such statements true. What feature of reality grounds the truth of "it's possible that P"? As we saw with Kripke semantics, a powerful way to analyze this is to say:

• "P is possible" is true if and only if there is some possible world w where P is true.
• "P is necessary" is true if and only if for all possible worlds w, P is true at w.

This is a powerful semantic analysis, but it just pushes the metaphysical question back a step. What are these "possible worlds" that our modal operators are quantifying over?

2. Modal Realism: Worlds as Concrete Universes

The most provocative and, in some ways, most straightforward answer comes from the philosopher David Lewis. His position, known as modal realism or concretism, asserts that other possible worlds are not abstract representations but are concrete, physical realities, just as real and robust as our own.

As you've just read, Lewis's view can be summarized by a few key claims:

1. Worlds are Concrete: Other possible worlds are vast physical systems, unified by spatiotemporal relations, just like our universe. They are not just sets of sentences or abstract ideas.
2. Parity of Reality: Our world and all other possible worlds are equally real. They do not differ in their kind of existence.
3. Spatiotemporal Isolation: These worlds are completely isolated from one another. You cannot travel from one to another, and events in one world cannot causally affect events in another. This distinguishes Lewis's view from the "many-worlds" interpretation of quantum mechanics, where universes can split.
4. "Actual" is an Indexical: The word "actual" does not name a special property that our world has and others lack. It functions like the words "here" or "now." When I say "this is the actual world," I am simply pointing to the world I inhabit, just as when I say "it is now noon," I point to the time of my utterance. Someone in another possible world is correct when they claim their world is the actual one.

The Argument from Theoretical Utility

Why should we accept such an ontologically extravagant theory? Lewis's main argument is one of theoretical utility or serviceability. He argues that the existence of a plurality of worlds, despite its strangeness, is a hypothesis that pays for itself by solving a vast range of philosophical problems with unmatched power and simplicity. It's an "inference to the best explanation."

This style of argument should be familiar from your physics background. We believe in unobservable entities like quarks or fields not because we see them directly, but because positing their existence provides the best explanation for the phenomena we do observe. Lewis asks us to treat possible worlds the same way.

Transworld Identity and Counterpart Theory

A major challenge for any theory of possible worlds is accounting for statements about what specific individuals could have been. When we say "You could have been a chemist," we seem to be talking about you in another world. But if you exist in another world and are a chemist, you would have different properties than you actually do (e.g., a different academic history). This violates a principle known as the Indiscernibility of Identicals (if X and Y are identical, they must share all the same properties). How can you be identical to the chemist-you if your properties differ?

Lewis's solution is radical. He denies transworld identity. You do not exist in any other possible world. Instead, other worlds contain your counterparts: individuals who are not you, but who resemble you in relevant and important ways. What makes it true that "you could have been a chemist" is that some counterpart of yours in another world is a chemist.

This is perhaps the most debated part of Lewis's theory. Critics, like Saul Kripke, argue that this changes the subject. We want to know about our own possibilities, not the careers of people who merely resemble us in other universes.

3. Actualism: The Primacy of the Actual

The "incredulous stare" that Lewis's theory often provokes leads us to the opposing camp: actualism. Actualism is the doctrine that everything that exists is actual. There are no "merely possible" objects or non-actual worlds existing in the same concrete way as our own. Reality is limited to the actual world and its contents.

This aligns with common sense, but it leaves the actualist with a difficult task: if other worlds aren't concrete places, what are they? And how do we account for possibilities involving things that don't actually exist, like your possible children or a species that never evolved?

The general actualist strategy is to claim that possible worlds are abstract objects that exist in the actual world. They don't exist alongside our world; they are part of its abstract furniture, like numbers or sets. This view is often called abstractionism or, in Lewis's slightly dismissive terminology, ersatz modal realism (ersatz meaning "substitute").

As this reading makes clear, abstractionist views come in several flavors, but they share common features that contrast sharply with Lewis's realism:

1. Worlds are Abstract: A possible world is an abstract entity, such as a maximal consistent set of propositions, or a maximal possible "state of affairs" (a concept championed by Alvin Plantinga). A world is a complete description or representation of a way reality could be, not the reality itself.
2. Irreducible Modality: Crucially, these accounts define worlds using modal terms. A world is a maximal possible state of affairs. This means abstractionism doesn't provide a reductive analysis of modality in the way Lewis claims to. It takes modality as a primitive feature of reality and uses it to explain what worlds are, rather than the other way around.
3. Transworld Identity: Most actualists accept that the very same individual can exist in multiple possible worlds. When we say "You could have been a chemist," we are talking about you, existing in another abstractly represented world with the property of being a chemist. This avoids the controversial counterpart theory.

The Problem of "Merely Possible" Individuals

The most pressing challenge for actualism is to account for possibilities involving things that don't actually exist. Consider the true statement: "It is possible that I have a sister" (assuming I don't).

• For Lewis, this is easy: in another concrete world, a counterpart of my parents has a daughter.
• For an actualist, this is hard. If only actual things exist, there is no "merely possible sister" for the proposition to be about. The world seems to lack the required truthmaker.

The most influential actualist solution to this problem is Alvin Plantinga's theory of haecceities (from the Latin for "this-ness").

Plantinga's clever move is to posit an ontology of abstract properties that exist necessarily, even if they are not exemplified by any concrete object.

• An essence of an object is a property that it has essentially (in every world where it exists) and that nothing else could have.
• A haecceity is a "purely nonqualitative" essence, like the property of being Socrates.

For Plantinga, for every individual that could possibly exist, its haecceity exists actually and necessarily as an abstract object. So, while my merely possible sister doesn't exist, her haecceity—the property of being that specific person—does exist in the actual world. This haecceity serves as a "trace" or "proxy" in our world for the individual who would have existed in another. Thus, the statement "It's possible I have a sister" is made true by facts about these actually existing abstract objects (haecceities).

4. The Debate in Summary

We can now see the debate as a clash of philosophical methodologies and a series of profound trade-offs.

Here is a summary of the main points of contention:

Conclusion

The debate over the status of possible worlds lies at the heart of contemporary analytic metaphysics. It forces us to ask what we are willing to accept into our ontology and what we demand from a philosophical explanation.

Key Takeaways:

• Modal Realism (Concretism): Championed by David Lewis, this view holds that other possible worlds are concrete universes just like ours. Its main motivation is its immense explanatory power and its ability to provide a reductive analysis of modality. Its main cost is its extraordinary ontological commitment.
• Actualism (Abstractionism): This family of views, represented by thinkers like Alvin Plantinga, maintains that only the actual world exists. Possible worlds are abstract objects (like states of affairs) that represent ways the world could have been.
• The Trade-Off: Lewis offers a powerful, reductive explanation of modality at a high ontological price. Actualists maintain a commonsense ontology but must accept modality as a primitive, unanalyzable feature of reality and rely on complex abstract machinery (like haecceities) to account for de re possibilities.
• Key Concepts: You should now be able to explain the roles of indexicality and counterpart theory in Lewis's realism, and the role of states of affairs and haecceities in Plantinga's actualism.

This isn't just a technical debate. Your stance determines how you ground some of our most fundamental concepts: essence, causation, freedom, and the very nature of possibility itself.

Preview of the Next Module:

The questions we've grappled with—what is ultimately real? what are the fundamental constituents of the universe? how do abstract concepts relate to concrete reality?—are not new. For millennia, philosophers have offered competing answers. In our next module, we will journey back to the very beginnings of Western philosophy to see how these debates first emerged. We will start with the Pre-Socratic thinkers and their attempts to find order in the cosmos, before turning to the two titans of ancient thought, Plato and Aristotle, whose own theories about what is most real (Forms vs. Substances) set the stage for two thousand years of metaphysical inquiry.