To: Francis W.
From: Geoffrey Klempner
Subject: Are possible worlds really real?
Date: 24 November 2006 12:26

Dear Francis,

Thank you for your email of 19 November, with your first essay for the Possible World Machine, in response to the question, 'Are possible worlds really real?'

You have set about this in the right way, first raising the question, 'What is 'Real'?' and then applying the answer to the specific topic of possible worlds.

Like you, I am a fan of the Matrix and think that the discussion of 'reality' in the film is relevant to this question.

Is Morpheus right, that 'real' is 'simply electrical signals interpreted by your brain'?

The idea is that the signals are intrinsically the same, whether produced by perception of objects outside you, or via wires and electrodes attached to your sleeping body. Either way, you have the capacity to be an 'agent' - manipulating physical objects in the world, or virtual objects in the virtual world. However, Morpheus 'knows' the 'truth' which other inhabitants of the virtual world do not, that they are merely asleep, being used as Duracells.

Or does he?

Morpheus could still be wrong about what he takes to be 'real' - a possibility explored in another film with a similar theme, Existenz. In that film, the main character wakes up - only to discover that he is in another dream. No experience of 'waking up' can prove that you are now in the 'real world' and not still asleep. If you have every had a nightmare where you thought you woke up, you will know what I mean.

The question, 'What is real?' is not equivalent to the question, 'How do we know what is real?' Even if we cannot prove that we are not asleep, we still know what we mean by the term 'real'. This is how scepticism about the external world is possible. The sceptic exploits the fact that we 'know what we mean' but don't know whether the possibility is realized or not.

Descartes raised the same question, but he thought the question through further than Morpheus. We think we know what we mean by 'real' - existing as a physical agent in direct contact with the physical objects which one encounters in experience, rather than asleep in a pod - but how do we know that ANYTHING physical exists? Descartes considered the possibility of a world which is not physical or spatial, where all that exists is me and an Evil Demon who creates dreams of a world of objects in space and time in my non-physical soul. All I know is that I exist, mentally. I do not know that anything physical exists.

In your essay you make the important point that what is 'real' is what 'really effects our actual feeling and our conscious experience'. In other words, in the Descartes scenario, what is real is the evil demon. In the Matrix, what is real is the world where people are kept in pods. 'Real' is the label for whatever causal explanation of our experience is TRUE - even if we do not possess the means to discover with certainty which is the 'true explanation'.

How does this apply to possible worlds?

What your later paragraphs bring out is that there are two ways in which we might apply the term 'real' to possible worlds. Constructed worlds, like the worlds of video games are very 'real' to the persons who experience them, in the sense that they have a powerful causal effect on our experience and indeed the quality of our lives: for example, the youngster who beats someone up after playing a shooter game for several hours. What actually caused this behaviour was not a 'world' but a computer program which simulated a world.

So too, the 'worlds' described by novelists can have an immense effect on a reader, but what actually causes this effect is the thought and the words, rather than an actually existing 'world'.

In this sense, it seems that possible worlds cannot be really 'real'. Possible worlds do not have causal effects on our experience, because what causes the effect is the computer program, or the book.

However, it could be argued in reply that this merely shows that 'having a causal effect on experience' is too narrow a definition of 'real'. Someone who believes in the 'reality' of possible worlds - for example, the American philosopher David Lewis ('Counterfactuals', 'On the Plurality of Worlds') - would argue that if statements about 'What might have been' can be true, then we must believe in the 'reality' of anything whose existence is required to explain - in a logical way - how they can be true. According to Lewis, the only credible analysis of such 'counterfactual' statements involves reference to possible worlds. 'Therefore' possible worlds are real - for just the same reason that if statements about numbers can be true, then numbers are real.

David Lewis thinks that possible worlds are as 'real' as the actual world, the only difference between the actual world and other possible worlds being one of perspective, like the difference between 'then' and 'now' or between 'you' and 'I'.

However, in the unit, the character Lucy suggests a less extravagant alternative to this 'all or nothing' view of the reality of possible worlds:

'Anything we talk about has to be real in some sense or else we couldn’t talk about it. Agreed? For example, Sherlock Holmes is a real character of fiction. He exists in the pages, so to speak, of the books Sir Arthur Conan-Doyle wrote. Now, possible worlds don’t have to be "real" in the way this actual world is real, if you see what I mean, any more than fictional characters are supposed to be actual living people. But these other worlds do have the kind of reality that applies to things of that sort, I mean, the kinds of things that we think about as being possible rather than actual. There’s nothing more to say!'

Is there? - I'm hedging my bets.

All the best,

Geoffrey