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On Ch. 4 of Lewis's book Counterfactuals (1973) and the essays “Scorekeeping in a Language Game” (1979) and “Truth in Fiction” (1978).
What makes counter-factual statements true? If you think "I might have grown up in Cleveland" is true, then what thing about the world makes that "might" statement true? Or by contrast, "I might have been a round square" is not only obviously false, but it can't be true. We could say that it's true that "it's impossible for a round square to exist," or to put the same point another way, "it's a necessary truth that nothing can be both round and square." Some philosophers going back to Leibniz have described these modal notions of possibility and necessity in terms of possible worlds. Something is necessary (e.g. maybe 1+1=2) if the proposition expressing it is true in all possible worlds. Something is possible (might have been) if it's true in at least one world.
This idea was formalized (meaning converted to logical symbols) by a different Lewis, C.I. Lewis, near the beginning of the twentieth century, and further developed by Rudolf Carnap, Saul Kripke, and others. David Lewis is perhaps best known for taking this notion metaphysically seriously, saying that possible worlds aren't just a metaphor but are metaphysically real. Matt Teichman from the Elucidations podcast joins Mark, Wes, and Dylan to figure out what this might mean. Clearly Lewis doesn't mean that they're part of the "actual" world, i.e., our world. They're merely possible. If you were living in one of those possible worlds (and technically, you couldn't, i.e., the actual you; at best a counterpart of you could exist in a possible world), then you'd consider that world actual and ours merely possible. So while ordinary language considers "real" and "actually existing" to be synonymous, Lewis is separating those.
We do our best to use the work of our previous episodes to make sense of all of this. In considering first Quine's "On What There Is" in ep. 66 and then Carnap's response in "Empiricism, Semantics, and Ontology" in ep. 191, we came across the idea that "to be is to be the value of a bound variable." This means that when we talk, we use expressions like "everything," and we should then look at what "things" are covered by that "everything" to decide what our ontology (the list of types of things that we think exist) is. So if you want to make statements about all numbers (and most mathematicians do!), then in some sense numbers have to be in your ontology, or maybe you have to engage in some elaborate reduction to show that (as an empiricist like Quine would prefer), no, numbers are just abstractions from macroscopic things in the actual world.
Lewis is just applying this insight about numbers to possible worlds. To do math, we need to talk about numbers. To use modal concepts like "possible" and "necessary," we need to talk about possible worlds (or more precisely, talking about possible worlds makes sense of the common-sense modal intuitions that result in us using these terms). Lewis considers various ways that possible-world talk could be reduced to other sorts of talk (maybe a possible world is just a consistent set of propositions), but he finds those alternatives more problematic than just talking about possible worlds.
But what is it to insist that talk of these worlds is not just irreducible to some other kind of talk, but possible worlds themselves are real? What does "real" add to this? Reflecting on our series from 2018 on the notion of truth (starting with ep. 194), we decide that at least at this stage of Lewis's thinking, maybe we can say that saying it's "real" doesn't actually add anything to saying that it's an irreducible kind of talk. This is a deflationary theory of what it is to be real (or true). For Tarski (and further, back to Frege), saying "it is raining" and "it is true that it is raining" are the same; "it is true that" adds nothing. So "we need to talk about possible worlds to make sense of our modal intuitions" and "we need to talk about possible worlds to make sense of our modal intuitions, and furthermore, possible worlds are real" are two sentences with the same logical content in them, even though they look different.
Note that in order to find possible-world talk useful, you don't have to agree with Lewis about their metaphysical status. In part two of our discussion, we'll get into how this possible-world talk applies to the philosophy of language. Don't wait for it; get the full, ad-free Citizen Edition now. Please support PEL!
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Image by Solomon Grundy.
Nathan Likert says
April 9, 2020 at 11:02 amReally enjoyed this – been looking forward to you guys tackling Lewis for a good while. Early modern hobbyist here, so I can chime in on Leibniz. As you guys mention, for Leibniz possible worlds do have a genuine existence, but only in the mind of God before the Creation. For Leibniz a world is a totality of maximally-consistent entities (i.e. that don’t violate the laws of logic to exist together) – his term for this is “compossibility.” The key consequence is that any being or action or phenomenon thus implies the entire rest of the universe: if I buy pecan rather than almond ice cream, that single choice entails an entire universe of compossible phenomena to allow and accomodate that event. This is how he gets monads: every monad or substance has a “complete notion” which includes all its predicates, everything that will ever happen to it and ALSO everything that happens in the entire universe, which it “reflects.” Monads and worlds imply each other, in other words (which also entails his famous pre-established harmony: since all monads express the universe, but this expression comes only from within, there’s so such thing as genuine relations, but at the level of the universe itself all the individual monads are coordinated. It’s like everyone is in a different windowless room but watching the same movie, just from different perspectives). Anyway, what’s really fascinating about worlds, as Matt touches on, is that God is seemingly forced to choose the best one (that is, the one that has max phenomena with minimum laws) – He has the attribute of goodness, so his choice is basically circumscribed from the beginning (Leibniz is an intellectualist, for whom God’s intellect takes priority over his will – Descartes on the other hand is a voluntarist – for him even the truths of mathematics depend on God’s will). Leibniz waffles on this: sometimes he describes it from the perspective of God as a divine calcuator (as in the “Discourse on Metaphysics”), but sometimes from the point of view of the worlds themselves, as though they’re putting themselves forward to be created (this is the “striving possibles” theory, in “On the Ultimate Origination of Things”). So the upshot: worlds only exist in God’s mind, and the best one gets actualized, so it’s the only real world. BUT, because God didn’t technically have to actualize that world (even though it was clearly the best: he could have just chosen to not create at all), everything in this world remains contingent. So a contingent proposition for Leibniz is any empirical phenomenon at all, even though it’s “also” in a sense necessary because this is the only actual world and all the beings and actions in it imply each other – the possibility from before the Creation carries over into this world, making the ENTIRE world contingent. This is also how he gets a compatibilist free will – we couldn’t have chosen otherwise in this world but this whole WORLD where I chose this didn’t have to be here. Hopefully that’s helpful! So he’s not all that Lewisian, in other words.
Steven S. says
May 29, 2020 at 6:22 amNothing like listening to this podcast while quarantining on a balcony looking up at a parade of passing clouds :- ) . Something that struck me: every time you all said a subjunctive, like “would have been” or “could be” and then spoke of those possibles as real or existing, it seemed that the subjunctive undercut the claim to real. Either you “do” have sister or you don’t. If she has died we say you “had” a sister. For you to “might have” a sister is a shift to your knowledge of the state of affairs, not a claim to the existence of the sister in question. How is this inevitable gap between the existing world and the known world closed”? It never completely can be, but observation, discussion, eventual agreement or dismissal by the “community of inquirers” (C.S.Peirce) is all we have. In some ways, Lewis points to the dangers of the analytic method to be too cute by half; pragmatism (and Occam’s razor) help prevent this. But you could mention Whitehead as another kindred soul in some ways?