Philosophy
The Metaphysics of Possibility and Actuality
Quick fact
The philosopher David Lewis argued for 'modal realism': the view that every possible world—every way the world could be—is as real and concrete as our own universe.
Why this is interesting
You know the world could have been different—but what does that mean? Is that other world real somewhere, or is possibility just a feature of our own?
Read the full explanation
Understanding The Metaphysics of Possibility and Actuality
Imagine you're planning a trip. You think, 'I could fly to Paris or I could take the train.' In ordinary talk, 'could' expresses possibility. Philosophers ask: what makes that 'could' true? For centuries, they debated whether possibility is just a mental construction or something about reality. In the 20th century, a powerful tool emerged: 'possible worlds.' A possible world is a complete way things could be. Our world is one such world—the actual world. Other possible worlds are different ways things might have been. So, 'I could have flown' means there is a possible world where I fly. 'Necessarily, 2+2=4' means it's true in all possible worlds. This framework lets us analyze statements about necessity and possibility rigorously. But it raises a deep question: what kind of existence do these other worlds have? Are they just abstract representations, or are they as real as our own? That's the heart of the metaphysics of possibility and actuality.
A deeper explanation
The central divide is between two broad camps. First, 'actualism': the view that everything that exists is actual—that there is only one real world, our own. On this view, talk of possible worlds is just a useful fiction or a way to describe patterns in the actual world. Possible worlds might be abstract entities like sets of propositions, or they might be purely linguistic devices. Actualists say that 'there is a possible world where I am a pianist' merely means 'it is consistent with the actual facts that I am a pianist.' The second camp, 'modal realism,' is defended most famously by David Lewis. Lewis argued that possible worlds are concrete, spatiotemporal universes, causally isolated from our own. They are not abstract or fictional; they exist just as much as our world. This view has startling implications: every way the world could have been is a real place. Why would anyone believe that? Lewis argued it gives a simple and powerful account of modal truths: necessity is truth in all worlds, possibility is truth in some world, and counterfactuals are evaluated by the closest possible worlds. It also avoids the problem of why some merely possible things 'exist' in a thinner sense. But modal realism clashes with our intuition that only the actual world is real. Actualists counter that we don't need to populate reality with countless worlds to explain modality; we can ground it in the nature of the actual world itself—for instance, in the powers of things or in logic. This debate illuminates a fundamental choice: whether modality is a fundamental feature of reality or a product of our own concepts and language. Understanding this choice helps us see what is at stake when we say something 'must' or 'might' be the case.