The American philosopher David Lewis (1941 – 2001) argued that, beyond this world, the actual world, there are countless possible worlds of a certain nature. These possible worlds exist and are as real as the actual one. The inhabitants of this world are only a few compared to all the inhabitants of all the worlds.
For Lewis, a world is defined as the totality of its contents: the Big Ben and the Andromeda Galaxy are as much part of this world as a brontosaurus and the next Pope. He says: “(…) nothing is so far away from us in space, or so far in the past or the future, as not to be part of the same world as ourselves” (1986, p. 70). This is to say that a world is made up, and unified, by all the things which are spatiotemporally connected. Lewis calls these things ‘worldmates’ (1986, p. 71).
On Lewis’ account, each world is completely separate from other worlds. There are no spatial, temporal or causal connections between one world and another (Talbot 2014). They may all belong to the same logical space but they are all sealed off. Worlds don’t leak or overlap. Unlike in popular fiction, there cannot be any gateway between various worlds. As a result, each worldmate can exist in one world only (Weatherson 2016, chapter 6.1).
Importantly, Lewis claimed that there are no logical gaps, meaning that every way a world can be some world is: “If there are many worlds, and every way that a world could possibly be is a way that some world is, then whenever such-and-such might be the case, there is some world where such-and-such is the case” (1986, p. 5). This means that there are worlds in which Brexit never happens and a Chinese astronaut is the first to set her foot on Mars. Or, there are worlds with different physical constants and natural laws in which galaxies never formed and life never emerged. Or, closer to home, there is another world in which David Lewis’ counter-part is still teaching philosophy at Princeton University.
Facts about what is actual are facts about how things are. Facts about modality are facts about how things could have been, must have been, or could not have been, in other words what is possible, necessary, or impossible (Vaidya 2017). Following Saul Kripke (b. 1940), Lewis argued that modal facts are facts about possible worlds. In keeping with Kripke’s modal logic, a statement like “possibly, p” is true if and only if there is at least one possible world in which p obtains. For example, the sentence “it is possible for me to be a rock-star” is made true by the fact that I am a rock-star in one or more possible worlds. On a similar note, the sentence “necessarily, p” is true if and only if p obtains in all possible worlds. Or, “it is impossible that p” means that p does not obtain in any world (Parent). Kripke, as the logician that he is, viewed possible worlds merely as abstract entities of formal logic. Lewis, as a metaphysicist, on the other hand, is a realist about possible worlds, a modal realist. For him, possible worlds are really existent worlds.
Lewis’ theory has led to a series of objections, only three of which shall be looked at here: First, talk of parallel worlds can be said to be nothing more than metaphysical speculation. From an epistemological point of view, one can never know if they exist or not. The theory cannot be tested. On a similar note, Richard argues that, since worlds are sealed off, it is impossible to determine whether something is true or not in another world. “Unless it is true in my world, direct inspection is ruled out” (Richards 1975, pp. 109-110). Second, modal realism implies an overabundance of worlds, perhaps infinitely many. It is conceivable that there are possible worlds such that one consists of only one particle, another one of two particles, and yet another one of three particles, and so forth, with the numbers of particles converging to infinity, yielding a heavily overcrowded logical space with all sorts of worlds. Ontologically speaking, modal logic comes at a high price and it is debatable whether it is worth paying that price. Third, Lewis’ theory clashes with common-sense. The idea that there are many worlds inhabited by one’s counterparts is deeply unsettling and contra-intuitive. It also raises serious questions about morality. Why, for instance, should one make an effort to save the child from drowning? Given the possibility that the child drowns, it does not matter if the child drowns in the actual world or one of the child’s counterparts drowns in another. “(…) from the standpoint of the universe it should make no difference which world is actual” (Blackburn 2016, p. 309).
So, despite these criticisms, why believe that possible worlds do really exist? Lewis claims: “Modal realism is fruitful; that gives us good reason to believe that it is true” (1986, p. 4). A good illustration of the philosophical utility of Modal realism is the way it solves the problem of counterfactuals. Counterfactuals appeal to what might have been. They take the form of conditional sentences in which the antecedent is false (Talbot). For example: If there hadn’t been a mass extinction 66 million years ago, dinosaurs would still be roaming the Earth today. Many would agree that this proposition is true. However, it is entirely unclear why. For all that is known, there was a mass extinction 66 million years ago. Hence, there is nothing in this world that makes the proposition true. This is where Lewis’ modal realism comes in: by postulating that there is another world in which there was no such mass extinction and in which dinosaurs still exist a truth-maker can be successfully provided. Another example of the philosophical utility of modal realism is its explanatory power when it comes to the existence of non-actual objects. Lewis holds that there is no difference in the being of actuals and non-actuals. This is owed to the fact that the term ‘actual’ is merely indexical. Like ‘here’, ‘now’ and ‘I’, the referent varies according to the context in which these words are uttered. In this sense, the term ‘actual’ describes the location of an object, e.g. of a brown duck, in this world. By contrast, a non-actual object such as a blue duck would exists in a world other than the actual world. Hence, modal realism allows for the reduction of the different types of being to just one: the actual brown duck and non-actual blue duck exist in the same kind of way, just in different worlds. This reduction in kinds provides a limited number of ontological primitives which is greatly valued by metaphysicists.
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