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Are possible worlds really 'real'?


To: Graham H.
From: Geoffrey Klempner
Subject: Are possible worlds really 'real'?
Date: 1st April 2011 13:52

Thank you for your email of 23 March, with your first essay for the Possible World Machine, in response to the question, 'Are possible worlds really real?'

You focus on the question of the utility of talk of possible worlds. Although some contemporary philosophers talk disparagingly of the use of thought experiments as methods fo exploring truth, this criticism does not in itself bear on the question whether possible worlds are 'really real' or not. Even if possible worlds are linguistic constructions, it is at least arguable the exploration of the properties of these constructions could still provide knowledge, the kind of knowledge that the philosopher seeks. I happen to believe this, and in this program you will find a number of examples where this method is used.

So what would be an argument relevant to the question whether possible worlds are 'really real' or not? And, in any case, what exactly does that mean?

You talk about the 'multiverse', a notion which has come into prominence with the many-worlds interpretation of Quantum Mechanics. I think there is an important difference between the multiverse theory and the strongly realist view of possible worlds. All the different 'worlds' existing in the multiverse came into existence through a process of branching. Whenever there is a quantum event, such as an electron going from a lower energy level to a higher energy level, two 'worlds' come into being, the world where the electron changes energy levels at that precise time, and the world where it does not change levels at that time. However, it would be consistent with the multiverse view to deny that *any* arbitrary possible world that I might think of belongs in the multiverse, e.g. the possible world where I have two heads, or, more soberly, the possible world where the orbits of Jupiter and Saturn are switched. We don't know whether or not there exists a possible sequence of branching events that would get you there. The point about possible worlds is that any world you can think of in a logically consistent way is a possible world.

You refer to David Lewis who is 'well known for his extreme position' According to Lewis, each possible world exists in its own space and time. The difference between what we term the 'actual world' and other possible worlds amounts to nothing more than a difference in local perspective, in a somewhat similar way to the difference between the perspective of GH and the perspective of GK (the same actual world seen from a different points of view) or the difference between a given historical fact, say the bombing of Hiroshima, as seen from the perspective of 1 April 1951 and the perspective of 1 April 2011. (You'll find some thoughts about this in the final chapter of my book 'Naive Metaphysics' downloadable from the Pathways site at

Lewis doesn't rest his argument on the analogy with the perspective of a given person or a given time, although these provide a kind of indirect support for the idea. If possible worlds were really real, then this is how we would be able to think about them.

So what is the argument? The argument, which is essentially the argument Lewis gives, is given a brief airing in the unit. Here's Brenda:
'Look, are we all agreed that when we say things about possible worlds, that is, when we talk about what might have happened if such-and-such had happened, we intend to say something true? Dr Phillips?'


'And, similarly, don't we sometimes make the claim that certain things or situations are, in themselves, possible or impossible, and intend such claims to be true?'

'OK. So what?'

'I don't see where this is going.'

'It's very simple, Derek. What we can or cannot imagine, or what we think about possible worlds, is not what makes those worlds real. Because we can be wrong. Our thoughts about possible worlds are true or false depending on something -- whatever it is -- that is somehow independent of those thoughts. What makes possible worlds real, in other words, can't simply be our thinking about them. Our minds discover something that has a reality independent of our minds.'
In more technical terms, Lewis's argument hinges on the question of the truth conditional semantics of counterfactual conditions, conditional statements of the form, 'If such and such had been the case, then so and so would have been the case.'

Counterfactual statements are used in science. 'If the temperature had gone below 0.002 degrees K, the conductivity of the sample would have increased by a factor of 30.' According to our best theory, say, this statement is true, even though we don't have the means to reduce the temperature to 0.002 degrees K. (I'm making these figures up, I don't know how realistic they are.) So what does it mean to state that this counterfactual statement *states a truth*? In his book, 'Counterfactuals' Lewis argues that his view is the only theory that adequately/ successfully accounts for the truth conditions of counterfactuals. The onus is on anyone who disagrees, to propose a better, ontologically more economical theory.

There is one other argument, which some find attractive: If it were really true that all possible worlds are real and that this world is nothing more than just another possible world, it would obviate any need to consider the question, 'Why does this world exist?' You don't need a Leibnizian God choosing which possible world to make 'real' if they are all equally 'real'.

All the best,