To: Bernd K.
From: Geoffrey Klempner
Subject: Are possible worlds really 'real'?
Date: 15th November 2011 13:21

Dear Bernd,

Thank you for your email of 7 November, with your first essay for Possible World Machine, in response to the question, 'Are possible worlds really 'real'?'

It's fair to say that you don't really answer this question, although you initially go about this in the right way. If you hadn't stopped when you exhausted the 800 word limit (which is in fact generously elastic, I should have told you, sorry!) I don't think from what you have indicated that you would have got there.

Just one point: it was Berkeley, not Descartes, who denied the existence of a material world of objects in space. Descartes raises the question of scepticism about an external world then answers his own question by means of the God hypothesis and the theory of mind-body dualism.

You are right that there is (at least in the current scientific climate) a strong disposition towards regarding questions about 'reality' as being measured by the standards of physical existence, dualism notwithstanding. However, what is interesting about the question whether possible worlds are 'really real' is that a materialist can happily debate this with a Cartesian dualist, neither having to give ground on their fundamental metaphysical theory. (Or with a Berkeleian idealist, for that matter.)

The key point, which you do raise, concerns relation to time. Even the 'worlds' generated in the many-world theory of QM trace back in a tree-like structure to specific points in time (e.g. an electron either does or does not fall into a lower orbit releasing a photon). Whereas, according to David Lewis, the chief proponent of the 'really real' theory of possible worlds (in his books 'Counterfactuals' and 'On the Plurality of Worlds') possible worlds each exist entirely in their own space and time with no point of contact with the actual world.

It makes to sense to ask the question 'when' the possible world where GK decided to take the day off today 'occurred'. It is not as if somewhere, right now, a counterpart of GK is enjoying a late, lazy breakfast. This is something I might have done, despite the pile of work on my desk, but wisely didn't do.

In the 'larger world of memories and imagination' (your last sentence) worlds exist, but these are just imagined, made up, constructed. If this is how we should understand the ontology of possible worlds, then that is tantamount to saying that possible worlds are not really real. They are not real at all. They exist only in the imagination.

Now one can debate whether, e.g. unicorns or Santa Claus exist 'only' in the imagination, or whether they have a more solid, cultural basis. There's all the difference in the world between one person's overheated imagination, and the shared memories/ imaginings of a significant portion of the human race. But still they are not 'real'. A unicorn, it is said, can detect whether a woman is a virgin or not. You don't need to undertake a scientific test for this, because this is true simply by virtue of myth and fable.

David Lewis is not being fanciful when he asserts that possible worlds are not like unicorns or Santa Claus (although there are possible worlds where 'unicorns' or 'Santa Claus' do actually exist). The existence of possible worlds follows as a logical/ metaphysical principle, if you accept a plausible view about the nature of language, viz. that the meaning of a statement is given by its truth conditions (a view first formulated by the great German mathematician Gottlob Frege).

It is the argument which is put into the mouth of Dr Phillips' student Brenda, in unit 1: '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.'

When I assert, 'There is a mug of lukewarm coffee on my desk,' my statement has truth conditions, it is true if the facts are a particular way, and false if the facts are a different way. I'm telling you that it is true (you have to believe me). It is made true by the thing on my desk which is, in fact, a mug of lukewarm coffee.

When I assert, 'If I had taken the day off today I would have enjoyed a lazy late breakfast', my statement according to David Lewis also has truth conditions. But what could these be? What makes it true, if it is true, that I would have had a late breakfast rather than setting off at the crack of dawn with my camera? Lewis's answer: The similarity of possible worlds makes it true. In the 'nearest', most 'similar' possible worlds to the actual world, I enjoy a lazy breakfast. That's a fact, just like the fact about the mug of coffee. If it is a fact, then possible worlds must be 'really real'.

How good is that argument? Where's the loophole? Surely, we don't want to be lumbered with this massive metaphysical extravagance! Or is it? What would Occam say? But then he would owe us an alternative, workable account of the semantics for counterfactual conditionals.

All the best,

Geoffrey