Philosophy
The Logic of Modal Realism and Concrete Possible Worlds
Quick fact
Lewis's theory is often called 'modal realism' because it treats the modal operators 'necesarily' and 'possibly' as quantifiers over a vast plurality of concrete worlds. He famously compared our position to being in a universe that is one of many, much like a grain of sand on a beach—yet each world is fully real.
Why this is interesting
If there are infinitely many universes just as real as ours, then every way the world could have been is actually a way some world is. What would that mean for your choices and your reality?
Read the full explanation
Understanding The Logic of Modal Realism and Concrete Possible Worlds
Start with a simple idea: you could have worn a red shirt today. That seems true even though you wore a blue one. But what makes it true? Many philosophers say that there must be some kind of 'possible world' where you wore a red shirt. A possible world is a complete way things could be. Modal realism takes this literally. It says that all possible worlds are real, concrete entities—not just abstract representations. Our own universe is one of these worlds. Each world is a large physical object, and the objects in it are concrete, like tables, planets, and people. Why would anyone believe such a strange idea? Because it makes modal logic very simple. When we say something is possible, we mean that it exists in at least one possible world. When we say something is necessary, we mean that it exists in all possible worlds. The word 'necessarily' becomes a universal quantifier over worlds, and 'possibly' becomes an existential quantifier. So modal logic gets a neat semantics. But there is a catch: if all these worlds are real, then they are just as real as our universe. That seems to conflict with our ordinary intuition that only our universe exists. How could there be other universes? Lewis replies that they are not connected to us—they exist, but they are far away in a different space-time. They are not observable from here, but they are not abstract. So modal realism is a bold, radical thesis about what exists. It says that reality is bigger than we thought. But it solves the problem of modal truth: every possibility is grounded in an actual, concrete world.
A deeper explanation
The mechanism behind modal realism is principled and economical: it analyses modality in terms of quantification over worlds. But to make this work, Lewis must specify what these worlds are like. He argues that each world is a sum of spatiotemporally interrelated objects. Different worlds are not spatiotemporally related to each other; they are isolated. The key principle is: 'Everything that exists is part of some world or other.' This includes all possible worlds, their parts, and the individuals within them. There is no world that is more real than another—our world is just one among many. This is the 'plurality of worlds' thesis. Why does this help logic? Consider the statement 'It is possible that there is a talking donkey.' Under modal realism, this is true because there exists a world (a concrete universe) that contains a talking donkey. Similarly, 'It is necessary that two plus two equals four' is true because in every possible world, two plus two equals four. So modal logic's semantics become straightforward. But this also leads to important consequences. For instance, because worlds are concrete, individuals only exist in their world. There is no trans-world identity—no object exists in more than one world. This creates a problem for the idea that you could have done otherwise: the 'other you' would be a different person (a counterpart) in another world, not the same you. This sparks the famous counterpart theory. Another consequence is that truth is world-relative. What is true in our world might not be true in another. It also raises questions about the nature of existence: if there are countless worlds, our own seems less special. Yet Lewis embraces this, arguing it is the price for a rich and elegant theory. In summary, modal realism works by taking possibility and necessity as literally describing parts of reality. It offers a clear mechanism: a modal claim is true just in case there exists (or there are) suitable concrete worlds. It reveals why modal logic can be so powerful, and it challenges us to rethink what we mean by 'real'.