Beyond Our Universe: Are All Possible Worlds Just as Real?
Our everyday experience paints a picture of a single, concrete reality – the universe we inhabit, filled with its particular set of laws, events, and entities. We often consider other possibilities as mere imaginings, counterfactual scenarios, or potential states of affairs that could have been but aren’t. However, a radical and mind-bending thesis in metaphysics challenges this deeply ingrained intuition: the idea that all possible worlds are just as real and concrete as our own.
The Core Thesis: Embracing a Multitude of Realities
This concept, most famously championed by philosopher David Lewis, proposes a view known as Lewisian modal realism. It asserts that every conceivable way a complete universe could have been – with different physical laws, different histories, different sets of entities – actually exists in its own right, just as tangibly and concretely as our own universe does. According to this view, there is no fundamental ontological difference between what we call the “actual” world and these “merely possible” worlds. They are all equally real.
“Actual” as a Matter of Perspective: The Here and Now of Existence
If all these possible worlds are equally real, then what makes our world so special, so “actual”? Lewis argues that “actual” is not an intrinsic property of our universe that sets it apart ontologically. Instead, it’s an indexical term, much like “here” or “now.” Just as “here” simply refers to the location of the speaker, and “now” refers to the time of the utterance, “actual” simply refers to the world from which the statement is being made or the world inhabited by the speaker. From the perspective of an inhabitant of another possible world, their world is “actual.”
Unpacking the Hypothesis: Possibility and Actuality Redefined
This framework leads to a reinterpretation of fundamental modal concepts:
Possibility as Existence Elsewhere: Venturing Beyond Our Cosmic Address
For something to be possible, according to modal realism, means that it exists or occurs in at least one possible world. If we can conceive of a universe where pigs fly or where the laws of gravity are slightly different, then such universes (and the flying pigs within them) actually exist somewhere within the vast totality of possible worlds. Possibility isn’t just a mental construct; it’s a matter of existence in another real, concrete realm.
Actuality as Our Local Reality: Anchored in Our Own Universe
Conversely, for something to be actual means simply that it exists or occurs in this world – the specific possible world that we, as speakers or thinkers, happen to inhabit. Our actuality is a matter of our indexical location within the pluriverse of all possibilities.
Shared Ontological Standing: Equal Partners in Existence
The crucial implication is that all these possible worlds, including our own, share the same fundamental kind of existence. They are not hierarchical, with our world being “more real” than others. From their own internal perspective, each possible world is just as “actual” as ours is to us. They are all equally concrete and equally real.
A Radical Departure: Challenging Common Sense
This view stands in stark contrast to our everyday intuitions and many traditional philosophical perspectives. Common sense tells us that our universe is the real one, and other possibilities are just that – possibilities, abstract ideas, or potential states that never came to pass. Alternatives to modal realism often treat possible worlds as abstract entities, mental constructions, or sets of consistent propositions, not as concrete universes existing alongside our own.
The Theory’s Purpose: A Powerful Tool for Philosophical Analysis
Lewis didn’t propose modal realism simply to be provocative. His primary motivation was to provide a powerful, systematic, and reductive analysis of modal concepts like possibility and necessity. By grounding possibility in actual existence in some possible world, he aimed to offer clear and concrete explanations for a range of complex philosophical problems, including the nature of counterfactuals (“what if” scenarios), the existence of properties, and the truth of propositions. For Lewis, embracing the reality of all possible worlds was a way to simplify and clarify some of the most challenging questions in metaphysics.
Delving into the Fabric of Reality: The Nature of Possible Worlds
To truly grasp the implications of Lewisian modal realism, it’s crucial to understand the specific characteristics attributed to these “possible worlds” that populate the pluriverse. They are not just vague alternatives; they are defined by specific ontological features.
Defining a “World”: A Self-Contained Spacetime
In the context of modal realism, a “world” is not simply a planet or a galaxy, but rather a maximal spatiotemporally interrelated whole. This means that everything that exists within a particular possible world is connected to every other thing in that same world through chains of space and time. Think of it as a self-contained universe, a complete spacetime domain.
The Isolation Principle: No Inter-World Travel
A key tenet of Lewis’s theory is that these possible worlds are spatiotemporally and causally isolated from one another. There is no physical travel, no direct communication, and no causal influence that can occur between distinct possible worlds. Each world operates according to its own internal laws and its own unique unfolding of events, entirely separate from the goings-on in any other possible world. Our universe, therefore, has no physical interaction with the universe where pigs fly or the one where dinosaurs never went extinct.
The Concreteness of Possibility: Worlds Made of “Stuff”
Unlike many other philosophical accounts of possibility, Lewis insists that possible worlds are concrete entities. They are not abstract sets of propositions, mental constructs, or merely potential states of affairs. They are just as real and made of the same fundamental kind of “stuff” as our own universe, even if the specific types of “stuff” and the laws governing them might differ from world to world. These are not shadowy afterthoughts of our reality; they are fully realized universes.
Maximality: Complete Ways Things Could Have Been
Each possible world is maximal or complete. This means that for any proposition P, either P is true at that world, or its negation (not-P) is true at that world. There are no incomplete or indeterminate possible worlds. Each one represents a total and specific “way things could have been,” a fully detailed history and configuration of reality.
The Principle of Recombination: Mixing and Matching Existence
How do these diverse possible worlds arise, with their varied contents and laws? Lewis proposes a “principle of recombination.” This principle suggests that possible worlds contain different arrangements and combinations of fundamental properties and individuals. Possibilities arise from essentially rearranging the basic “building blocks” that we find in our own actuality (or potentially across all possible worlds). The principle assumes that, with some constraints (like not combining properties in logically contradictory ways), parts of different possible worlds can be freely recombined to generate a vast landscape of possibilities. For example, if our world has electrons and unicorns are possible, then there’s a world with electron-unicorns (assuming no logical contradiction).
Excluding the Impossible: The Realm of Logical Consistency
Interestingly, and importantly, Lewis’s modal realism typically does not include logically impossible worlds – worlds where contradictions are true (e.g., a square circle exists). Because each possible world is maximal and governed by a consistent set of truths, logical impossibilities are generally excluded from the realm of genuine possibility within his framework. A scenario involving a square circle violates the very principles of logic that underpin the structure of these possible worlds.
By outlining these characteristics, we begin to grasp the sheer scale and the somewhat counterintuitive nature of the pluriverse as envisioned by Lewisian modal realism. It’s a reality far grander and more diverse than our single, familiar universe.
“Actual” as a Shifting Center: The Indexical Nature of Our Reality
One of the most crucial and perhaps initially perplexing aspects of Lewisian modal realism is its treatment of the term “actual.” To reconcile the idea of countless equally real possible worlds with our intuitive sense that our world holds a special status, Lewis proposes that “actual” functions indexically, much like words such as “here” and “now.”
Drawing Parallels: Location and Time as Analogies
Consider the words “here” and “now.” Their meaning is entirely dependent on the context of their use. “Here” refers to the specific location of the person speaking or writing at that moment. If someone in Tokyo says “here,” they are referring to Tokyo, not New York. Similarly, “now” refers to the specific time of the utterance. “Now” for someone in the 15th century refers to a different temporal moment than “now” for us in the 21st century. The meaning of these words shifts depending on who is using them and when.
Lewis argues that “actual” operates in a fundamentally analogous way. It doesn’t denote a unique ontological status or a special kind of reality. Instead, “actual” simply refers to the particular possible world that the speaker or the context of discussion happens to inhabit. When we say “the actual world,” we are merely picking out our world from the vast ensemble of all possible worlds, just as saying “here” picks out our current location.
The Absence of Absolute Actuality: A Relational Property
Just as there is no single location in the universe that is absolutely “here” – every “here” is relative to a specific point – so too, according to Lewis, there is no single possible world that is absolutely “actual.” Actuality is not an intrinsic property that some worlds possess and others lack. Rather, it’s a relational property, a matter of perspective. A world is “actual” for its inhabitants.
The Inhabitants’ Perspective: Our World as Just One Possibility
The logical consequence of this indexical theory of actuality is that when inhabitants of another possible world – a world with different laws of physics or a different history – use the word “actual,” they are perfectly correct in referring to their world. From their perspective, their universe is the “actual” one, the one they inhabit. Our world, with its particular set of facts and circumstances, is, from their viewpoint, merely one among the countless other possible worlds that exist. It holds no special ontological significance for them, just as their world holds no inherent “actuality” for us beyond being another existing possibility.
This indexical view of actuality is crucial for making modal realism coherent. It allows for the equal reality of all possible worlds while still accounting for our natural focus on and belief in the special status of our own experience. “Actual” becomes a pragmatic term, anchoring us in our local corner of the vast pluriverse.
“Paradise on the Cheap”: The Explanatory Power of a Multiverse
Despite its seemingly outlandish ontological commitment to a vast number of universes, Lewis argued that modal realism offers significant explanatory advantages, a kind of “paradise on the cheap” where a single, albeit large, ontological investment yields substantial philosophical returns by simplifying the analysis of numerous complex concepts.
Unlocking the Secrets of Modality: Possibility and Necessity Explained
One of the primary motivations for Lewis’s modal realism was its ability to provide a straightforward and non-circular analysis of modal concepts – possibility and necessity – which are fundamental to much of philosophy.
- A Clear Foundation: By positing the existence of all possible worlds as concrete realities, modal realism offers a clear and intuitive grounding for these concepts.
- Defining Possibility: Within this framework, the statement “Possibly P” (where P is some proposition) can be analyzed simply as: “There exists at least one possible world in which the proposition P is true.” For example, it is possible for a purple unicorn to exist because there is at least one possible world that contains a purple unicorn.
- Defining Necessity: Similarly, “Necessarily P” can be defined as: “The proposition P is true in all possible worlds.” For instance, if a logical truth like “A is A” holds in every conceivable way a universe could be, then it is necessarily true.
- Avoiding Circularity: This approach avoids the common philosophical challenge of treating modal operators like ‘possibly’ and ‘necessarily’ as primitive, undefined terms that we simply have to accept. Instead, it reduces them to the more fundamental notion of existence across a multitude of concrete worlds.
Illuminating Counterfactuals: What Might Have Been
Modal realism also offers a powerful framework for analyzing counterfactual conditional statements – “what if” scenarios that explore alternative possibilities to what actually happened.
- Worlds of Alternative History: A statement like “If P had occurred, Q would have occurred” can be analyzed as: “In the possible world or worlds that are most similar to the actual world in which the antecedent P occurs, the consequent Q also occurs.”
- A Basis for Similarity: While the notion of similarity between different possible worlds is complex and debated, modal realism provides a foundation for making sense of such comparisons. We can evaluate counterfactuals by looking at other concrete worlds that share a high degree of similarity with our own up to the point where the counterfactual antecedent (P) becomes true.
Understanding Properties and Relations: Instances Across the Pluriverse
Modal realism provides a novel way to understand the nature of properties (like being red, being tall) and relations (like being taller than, being next to).
- Properties as Sets of Instances: A property can be identified with the set of all its instances – all the individual things that possess that property – across all possible worlds. For example, the property of “being blue” is the collection of every blue object in our world, plus every blue object in every other possible world.
- An Extensional Account: This offers an extensional account of properties, defining them not by some abstract essence but by the totality of their concrete manifestations across the entire landscape of possibility.
Defining Propositions: Truth Across Worlds
Similarly, modal realism offers an analysis of propositions – the meanings of declarative sentences.
- Propositions as Sets of Worlds: A proposition, such as “snow is white,” can be identified with the set of all possible worlds in which that proposition is true. Our world is one of the worlds within that set.
Grounding Intentionality: The Content of Our Minds
The theory can even be applied to understanding the content of our intentional states, such as beliefs and desires.
- Mind as World-Selector: Beliefs and desires can be analyzed as relations between an agent and sets of possible worlds. For instance, to believe that it will rain tomorrow is to rule out all possible worlds where it doesn’t rain tomorrow (as far as your beliefs go). Desire can be seen as a preference for certain sets of possible worlds over others.
The Economy of Ontology: Simplicity in the Grand Scheme
Lewis himself argued for modal realism based on its overall theoretical utility and what he called “simplicity in ideology.” While the theory is undeniably ontologically extravagant in its postulation of countless universes, Lewis contended that it is qualitatively parsimonious. It allows us to reduce many different kinds of philosophical puzzles – concerning modality, counterfactuals, properties, propositions, and intentionality – to a single fundamental kind of entity: possible worlds and the concrete individuals and properties they contain. By making this one large ontological commitment, we can achieve a more unified and simpler overall theoretical framework (“ideology”) for understanding these diverse philosophical domains. The significant explanatory benefits, in Lewis’s view, justify the initial ontological cost. It’s “paradise on the cheap” because one big assumption buys us solutions to many problems.
The Price of Paradise: Objections and Challenges to Modal Realism
Despite the explanatory allure of Lewisian modal realism, it faces a number of significant objections and challenges that have led many philosophers to reject it in favor of more ontologically conservative accounts of modality.
The “Incredulous Stare”: The Burden of Ontological Extravagance
- The Sheer Multiplicity: The most common and visceral reaction to modal realism is often one of disbelief at the sheer ontological extravagance it entails. Positing an infinite or at least a vast number of concrete, causally isolated universes strikes many as wildly unparsimonious, directly violating the principle of Occam’s Razor, which generally favors theories with fewer entities.
- Plausibility and Evidence: Critics argue that such a massive commitment to unobservable entities seems scientifically implausible and entirely disconnected from empirical evidence. Unlike theoretical entities in physics that are often posited to explain observable phenomena, these other possible worlds are, by definition, causally isolated from our own, making direct or indirect empirical confirmation seemingly impossible.
The Epistemological Quandary: How Can We Know What We Can’t Touch?
- The Barrier of Isolation: If all these possible worlds are indeed causally isolated from our own, a significant epistemological problem arises: how can we possibly have any knowledge about them? We cannot observe them, interact with them, or receive any information from them.
- Lewis’s Defense: Lewis attempted to address this by arguing that our knowledge of other possible worlds comes via inference to the best explanation. Their existence, he claimed, provides the most systematic and coherent explanation for our modal intuitions and the analysis of concepts like possibility and necessity. Furthermore, he argued that we know what these worlds contain through the principle of recombination, which allows us to conceive of different arrangements of the fundamental properties and individuals we find in our own world.
- Skepticism Remains: However, many philosophers find this epistemological justification unconvincing. They argue that the significant ontological claim requires a stronger epistemic foundation than mere theoretical utility, especially when those entities are inherently unobservable and untestable.
The Identity Crisis: Trans-World Individuals vs. Counterparts
- Our Intuitive Modality: Our modal intuitions often involve the idea of the same individual existing in different possible scenarios. For example, we might say, “I could have been a doctor,” implying that the very same “I” exists in a possible world where I pursued a medical career.
- Lewis’s Rejection and Counterpart Theory: Lewis rejected this notion of “trans-world identity,” arguing that it leads to philosophical difficulties concerning essential properties and distinctness across worlds. Instead, he proposed “Counterpart Theory.” According to this view, individuals exist in only one possible world. When we talk about what “I” could have done, we are not referring to myself existing in another world, but rather to a counterpart in that world – an individual who is very similar to me in important respects (e.g., having a similar genetic makeup, similar early life events, etc.).
- The Subjectivity of Similarity: Critics argue that counterpart theory doesn’t adequately capture our intuitive understanding of modality. Talking about a similar counterpart isn’t the same as talking about what I could have done. Furthermore, the notion of “similarity” between individuals across different possible worlds seems vague and potentially arbitrary, undermining the purported clarity of the theory. It also raises questions about whether counterpart theory can adequately account for modal logic and de re modality (modal claims about specific individuals).
The Relevance of Isolation: Worlds Apart, Minds Together?
A further objection concerns the relevance of causally isolated entities to our understanding of counterfactuals and possibilities for us. If these other worlds have no causal connection to our own, how can they ground statements about what would have happened here if things had been different? It seems strange that entirely separate and unreachable universes should play a crucial role in determining the truth of claims about our own world’s alternative possibilities.
The Problem of Uniqueness: Infinite Near-Duplicates?
Modal realism, with its principle of recombination, seems to imply the existence of an infinite number of possible worlds, including many that are nearly identical to our own, differing only in the most minute details (e.g., a single electron being in a slightly different position). This raises questions about the distinctness of these worlds and whether the theory can adequately account for the uniqueness of our own experienced reality if there are countless near-indiscernible alternatives.
Moral and Existential Weight: Does the Pluriverse Dilute Our World?
Finally, some critics raise moral and existential concerns. Does the existence of infinite suffering or infinite happiness in other causally isolated worlds diminish the significance of events and experiences in our own? Does it affect our sense of responsibility for our actions in this world if there are countless variations occurring elsewhere? These are deep philosophical questions about the implications of such a vast and disconnected multiverse for our values and our understanding of our place in existence.
These objections highlight the significant hurdles that modal realism faces in gaining widespread acceptance within philosophy. While Lewis presented a powerful and systematic framework, the high ontological cost and the accompanying epistemological and conceptual challenges remain major points of contention.
Beyond Concreteness: Alternative Visions of Possibility
Lewisian modal realism, with its commitment to concrete possible worlds, is not the only way philosophers have sought to understand modality. Several alternative theories attempt to account for possibility and necessity without the vast ontological cost of a concrete pluriverse. Here’s a brief contrast with some prominent alternatives:
Possible Worlds as Abstract Entities: Ersatz Modal Realism and Abstractionism
The family of views known as ersatz modal realism or abstractionism agrees that possible worlds exist, but crucially, they posit that these worlds are abstract entities rather than concrete universes. These abstract entities can take various forms:
- Maximal Consistent Sets of Propositions (Plantinga): A possible world is a complete and consistent story about how things could have been, a set of propositions that are all true together in that world.
- Divine Ideas (Adams): Possible worlds are the blueprints or ideas in the mind of God, representing all the ways He could have created the universe.
- Structural Properties: Possible worlds are abstract structures or patterns of properties and relations.
Contrast with Lewis: The key difference is the ontological status of possible worlds. Ersatz realists avoid the commitment to countless concrete universes.
Challenges: These views face the “grounding problem”: how do these abstract entities (sets of words, ideas in a mind, structures) genuinely represent possibilities for a concrete world like ours? What makes a set of propositions about a world, and how does an abstract structure correspond to a way things could be concretely?
Possible Worlds as Useful Fictions: Fictionalism
Fictionalism, as advocated by philosophers like Gideon Rosen, takes a more deflationary approach. It suggests that talk about possible worlds is a useful fiction, a convenient way of analyzing modal concepts. We speak as if possible worlds exist to make sense of possibility, necessity, and counterfactuals, but in reality, they don’t have genuine ontological status.
Contrast with Lewis: Fictionalism denies the existence of possible worlds altogether, viewing them as mere tools for philosophical analysis.
Challenges: Fictionalism needs to explain why this “fiction” is so useful and systematic in analyzing modality. It also faces the challenge of providing a non-world-based account of what makes modal statements true or false. If there are no possible worlds, what grounds the truth of “possibly, it could have rained today”?
Possibility from Actuality: Combinatorialism
Combinatorialism, championed by philosophers like D.M. Armstrong, attempts to generate possibilities by recombining elements (individuals, properties, relations) that actually exist in our world. A possible world is then a specific combination of these actual ingredients.
Contrast with Lewis: Combinatorialism seeks to limit the ontological cost by grounding possibility in the actual world’s constituents, rather than positing entirely separate concrete universes with potentially novel entities.
Challenges: Combinatorialism may struggle to account for possibilities that seem to involve fundamentally new kinds of individuals or properties that are not found in our actual world. For example, how could we have genuine possibilities involving different fundamental physical laws or entirely novel particles if all possibilities are just recombinations of our current actual ingredients? It might also face difficulties in specifying the rules and constraints on how these recombinations can occur.
These alternative theories represent different strategies for grappling with the concept of possibility, each with its own set of advantages and disadvantages compared to the bold and expansive vision of Lewisian modal realism. They highlight the ongoing debate in metaphysics about the best way to understand the modal landscape.
The Ripple Effects of a Real Pluriverse: Consequences of Modal Realism
If Lewisian modal realism turned out to be the correct account of reality, the implications would be far-reaching, fundamentally altering our understanding across various branches of philosophy.
A Universe Multiplied: Reshaping Metaphysics
Perhaps the most striking implication would be a radical expansion of our metaphysical landscape. Reality would be vastly larger than the single universe we inhabit. Our own world would lose any privileged ontological status; it would simply be one among countless other equally real and concrete universes. This would necessitate a significant shift in our intuitive understanding of existence and our place within it.
Grounding the Possible: A Concrete Foundation for Modality
For the study of modality, modal realism would provide a concrete and seemingly straightforward foundation for concepts like possibility and necessity. “Possibly P” would no longer refer to some abstract potentiality but to the actual existence of a world where P is true. Similarly, “Necessarily P” would be grounded in the truth of P across the entire multitude of these concrete realities.
Meaning Across Worlds: A Framework for Semantics
Modal realism offers a powerful framework for analyzing the meaning of terms related to properties, propositions, and counterfactuals. Properties become sets of instances across all worlds, propositions become sets of worlds where they are true, and counterfactuals are evaluated by looking at the most similar worlds where the antecedent holds. This provides a systematic and unified approach to semantic analysis in these domains.
Mapping the Mental Landscape: Understanding the Mind
In the philosophy of mind, modal realism offers accounts of the content of our mental states. Beliefs and desires can be understood as relations to sets of possible worlds, providing a way to analyze intentionality and how our minds engage with possibilities beyond our immediate experience.
Rethinking the Self: Identity in a Multiverse
The rejection of trans-world identity and the adoption of counterpart theory would necessitate a significant rethinking of personal identity across possibilities. What it means for “me” to have been a doctor in another world becomes a question of similarity to a counterpart, rather than the numerical identity of the same individual persisting across different universes. This has profound implications for our understanding of who we are and what our potential futures (in other worlds) entail.
The Limits of Knowing: Epistemology in a Vast Reality
The causal isolation of possible worlds raises fundamental and challenging questions for epistemology. If we can never interact with or observe these other realities, how can we claim to have any knowledge about them? Lewis’s reliance on inference to the best explanation and the principle of recombination as epistemic justifications highlights the potential limits of traditional empirical methods when dealing with such a radically expanded ontology. It forces us to consider alternative modes of justification and the very nature of philosophical knowledge.
In essence, the truth of modal realism would usher in a dramatically different philosophical landscape, requiring us to reconsider our most basic intuitions about reality, possibility, meaning, identity, and knowledge. It would be a paradigm shift of monumental proportions.