physics

Modal realism

Modal realism is a physics topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Modal realism rather than just read about it. In short: Modal realism is the view propounded by the philosopher David Lewis that all possible worlds are real in the same way as is the actual world: they are "of a kind with this world of ours." It states that possible worlds exist, possible worlds are not different in kind from the actual world, possible worlds are irreducible entities, and the term actual in actual world is indexical, i.e. any subject can declare their w…

Key takeaways

Modal realism belongs to physics; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Modal realism to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Modal realism from memory before moving on to harder problems.

Reference excerpt

Modal realism is the view propounded by the philosopher David Lewis that all possible worlds are real in the same way as is the actual world: they are "of a kind with this world of ours." It states that possible worlds exist, possible worlds are not different in kind from the actual world, possible worlds are irreducible entities, and the term actual in actual world is indexical, i.e. any subject can declare their world to be the actual one, much as they label the place they are "here" and the time they are "now". Extended modal realism is a form of modal realism that involves ontological commitments not just to possible worlds but also to impossible worlds. Objects are conceived as being spread out in the modal dimension, i.e., as having not just spatial and temporal parts but also modal parts. This contrasts with Lewis' modal realism, according to which each object only inhabits one possible world. Common arguments for modal realism refer to their theoretical usefulness for modal reasoning and to commonly accepted expressions in natural language that seem to imply ontological commitments to possible worlds. A common objection to modal realism is that it leads to an inflated ontology, which some think runs counter to Occam's razor. Critics of modal realism have also pointed out that it is counterintuitive to allow possible objects the same ontological status as actual objects. This line of thought has been further developed in the argument from morality by showing how an equal treatment of actual and non-actual persons would lead to highly implausible consequences for morality, culminating in the moral principle that every choice is equally permissible.

"Possible world"

The term goes back to Leibniz's theory of possible worlds, used to analyse necessity, possibility, and similar modal notions. In short, the actual world is regarded as merely one among an infinite set of logically possible worlds, some "nearer" to the actual world and some more remote. A proposition is "necessary" if it is true in all possible worlds, and "possible" if it is true in at least one.

Main tenets At the heart of David Lewis's modal realism are several central doctrines about possible worlds:

Possible worlds exist — they are just as real as our world. Possible worlds are the same sort of things as our world — they differ in content, not in kind. Possible worlds cannot be reduced to something more basic — they are irreducible entities in their own right. Actuality is indexical. When we distinguish our world from other possible worlds by claiming that it alone is actual, we mean only that it is our world. Possible worlds are unified by the spatiotemporal interrelations of their parts; every world is spatiotemporally isolated from every other world. Possible worlds are causally isolated from each other.

Details and alternatives In philosophy, possible worlds are usually regarded as real but abstract possibilities (i.e., Platonism), or sometimes as a mere metaphor, abbreviation, or as mathematical devices, or a mere combination of propositions. Lewis himself not only claimed to take modal realism seriously (although he did regret his choice of the expression modal realism), he also insisted that his claims should be taken literally:

By what right do we call possible worlds and their inhabitants disreputable entities, unfit for philosophical services unless they can beg redemption from philosophy of language? I know of no accusation against possibles that cannot be made with equal justice against sets. Yet few philosophical consciences scruple at set theory. Sets and possibles alike make for a crowded ontology. Sets and possibles alike raise questions we have no way to answer. [...] I propose to be equally undisturbed by these equally mysterious mysteries. How many [possible worlds] are there? In what respects do they vary, and what is common to them all? Do they obey a nontrivial law of identity of indiscernibles? Here I am at a disadvantage compared to someone who pretends as a figure of speech to believe in possible worlds, but really does not. If worlds were creatures of my imagination, I could imagine them to be any way I liked, and I could tell you all you wished to hear simply by carrying on my imaginative creation. But as I believe that there really are other worlds, I am entitled to confess that there is much about them that I do not know, and that I do not know how to find out.

Extended modal realism

Extended modal realism, as developed by Takashi Yagisawa, differs from other versions of modal realism, such as David Lewis' views, in several important aspects. Possible worlds are conceived as points or indices of the modal dimension rather than as isolated space-time structures. Regular objects are extended not only in the spatial and the temporal dimensions but also in the modal dimension: some of their parts are modal parts, i.e. belong to non-actual worlds. The concept of modal parts is best explained in analogy to spatial and temporal parts. A person's hand is a spatial part of them just as their childhood is a temporal part, according to four-dimensionalism. These intuitions can be extended to the modal dimension by considering possible versions of the person which took different choices in life than they actually did. According to extended modal realism, these other selves are inhabitants of different possible worlds and are also parts of the self: modal parts. Another difference to the Lewisian form of modal realism is that among non-actual worlds within the modal dimension are not just possible worlds but also impossible worlds. Yagisawa holds that while the notion of a world is simple, being a modal index, the notion of a possible world is composite: it is a world that is possible. Possibility can be understood in various ways: there is logical possibility, metaphysical possibility, physical possibility, etc. A world is possible if it doesn't violate the laws of the corresponding type of possibility. For example, a world is logically possible if it obeys the laws of logic or physically possible if it obeys the laws of nature. Worlds that don't obey these laws are impossible worlds. But impossible worlds and their inhabitants are just as real as possible or actual entities.

Arguments for modal realism

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Worked examples

Example 1 — a first encounter with Modal realism

Start with the simplest possible case. Write down what Modal realism claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In physics, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Modal realism before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Modal realism ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Modal realism

In research: Modal realism appears in physics research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Modal realism in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Modal realism is common in secondary-school and first-year university syllabi. It links to neighbouring topics Analytic philosophy, Metaphysical realism, Modal metaphysics, so understanding it makes those chapters shorter.
In everyday life: Look for Modal realism outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Modal realism in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Modal realism means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Modal realism out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Modal realism in simple terms?

Why does Modal realism matter?

Because it connects several physics ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Modal realism?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Modal realism.