Modal realism

Term used for the theory, going back to Gottfried Wilhelm Leibniz (1646-1716), of ‘possible worlds’; used to analyze necessity and possibility and similar notions, which are known as modal notions.

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 statement is called necessary if it is true in all possible worlds, and possible if it is true in at least one. Possible worlds are usually regarded as real but abstract possibilities.

However, for David Kellogg Lewis they are concrete worlds, like this one only without any spatial or temporal connections with it.

Also see: counterpart theory

D K Lewis, On the Plurality of Worlds (1986)

The term 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. To note, however, though the concept does sound similar to modern notions of a multiverse, in reality these two concepts are quite distinct – a multiverse would constitute a single possible world.

Main tenets

At the heart of David Lewis’s modal realism are six central doctrines about possible worlds:

  1. Possible worlds exist – they are just as real as our world;
  2. Possible worlds are the same sort of things as our world – they differ in content, not in kind;
  3. Possible worlds cannot be reduced to something more basic – they are irreducible entities in their own right.
  4. 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.
  5. Possible worlds are unified by the spatiotemporal interrelations of their parts; every world is spatiotemporally isolated from every other world.
  6. Possible worlds are causally isolated from each other.

Details and alternatives

In philosophy possible worlds are usually regarded as real but abstract possibilities, or sometimes as a mere metaphor, abbreviation, or façon de parler for sets of counterfactual 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.[2]

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.[3]

Extended modal realism

Extended modal realism, as developed by Takashi Yagisawa,[4] 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.[5][6] My hand is a spatial part of myself just as my childhood is a temporal part of myself, according to four-dimensionalism.[7] These intuitions can be extended to the modal dimension by considering possible versions of myself which took different choices in life than I actually did. According to extended modal realism, these other selves are inhabitants of different possible worlds and are also parts of myself: modal parts.[4]:41[8]

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.[5][9] 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

Reasons given by Lewis

Lewis backs modal realism for a variety of reasons. First, there doesn’t seem to be a reason not to. Many abstract mathematical entities are held to exist simply because they are useful. For example, sets are useful, abstract mathematical constructs that were only conceived in the 19th century. Sets are now considered to be objects in their own right, and while this is a philosophically unintuitive idea, its usefulness in understanding the workings of mathematics makes belief in it worthwhile. The same should go for possible worlds. Since these constructs have helped us make sense of key philosophical concepts in epistemology, metaphysics, philosophy of mind, etc., their existence should be accepted on pragmatic grounds.

Lewis believes that the concept of alethic modality can be reduced to talk of real possible worlds. For example, to say “x is possible” is to say that there exists a possible world where x is true. To say “x is necessary” is to say that in all possible worlds x is true. The appeal to possible worlds provides a sort of economy with the least number of undefined primitives/axioms in our ontology.

Taking this latter point one step further, Lewis argues that modality cannot be made sense of without such a reduction. He maintains that we cannot determine that x is possible without a conception of what a real world where x holds would look like. In deciding whether it is possible for basketballs to be inside of atoms we do not simply make a linguistic determination of whether the proposition is grammatically coherent, we actually think about whether a real world would be able to sustain such a state of affairs. Thus we require a brand of modal realism if we are to use modality at all.

Argument from ways

Possible worlds are often regarded with suspicion, which is why their proponents have struggled to find arguments in their favor.[10] An often-cited argument is called the argument from ways. It defines possible worlds as “ways how things could have been” and relies for its premises and inferences on assumptions from natural language,[11][12][13] for example:

(1) Hillary Clinton could have won the 2016 US election.
(2) So there are other ways how things could have been.
(3) Possible worlds are ways how things could have been.
(4) So there are other possible worlds.

The central step of this argument happens at (2) where the plausible (1) is interpreted in a way that involves quantification over “ways”. Many philosophers, following Willard Van Orman Quine,[14] hold that quantification entails ontological commitments, in this case, a commitment to the existence of possible worlds. Quine himself restricted his method to scientific theories, but others have applied it also to natural language, for example, Amie L. Thomasson in her easy approach to ontology.[15] The strength of the argument from ways depends on these assumptions and may be challenged by casting doubt on the quantifier-method of ontology or on the reliability of natural language as a guide to ontology.

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