To: Ian W.
From: Geoffrey Klempner
Subject: Are possible worlds really real?
Date: 19 April 2002 10:45

Dear Ian,

Thank you for your e-mail of 6 April, with your first essay for The Possible World Machine in response to the question, 'Are Possible Worlds Really Real?'

I did check out your web site (I have been there before). I am pleased that you are posting your essays there. If you add a more explicit hypertext link to Pathways, I can put you on the Philosophy and Distance Learning Links page which (though it's hopelessly behind at the moment) contains all sites with a link to Pathways, large and small.

I really liked this piece.

There are two main issues to discuss. The first was deliberately left out of unit 1 because its ramifications are a bit too much for a first chapter, but what it comes down to is whether (a) you yourself exist in other possible worlds ('transworld identity'), or whether (b)what exists in other possible worlds are merely your counterparts ('counterpart theory').

(a) Saul Kripke, author of the ground breaking 'Naming and Necessity' (Blackwell) developed his philosophical view of possible worlds through his work with the semantics for modal logic. Consider the statement, 'Venus might have been the first planet from the sun'. How do we identify 'Venus' in another possible world? Kripke would say we look at the history of the formation of the *actual* planet Venus, and consider things that might have happened along the way which led to *its* being the first, rather than the second planet. E.g. Mercury exploded millions of years ago and its fragments were scattered, or the mass of material which formed into the planet Venus was flung out further into orbit while the mass of material that formed the planet Mercury was not flung out so far. In these alternative accounts, the actual planet Venus is identified in other possible worlds.

In a similar way, if you think what might have happened to you if you hadn't enrolled for Pathways, you identify the actual you in another possible world, by tracing a line back to the time when the actual you made the decision whether to enrol or not enrol.

(b) David Lewis, author of 'Counterfactuals' and 'The Plurality of Worlds' and foremost proponent of 'strong realism' about the existence of possible worlds takes a different line. Whichever planet we identify as 'Venus' in another possible world is the possible object that best fits the descriptions we associate with 'Venus' in the actual world. So, in the possible scenario where the positions of Mercury and Venus are switched, (and assuming that the look two planets to the naked eye is roughly the same) it is the second planet, not the first planet which would have been called 'Venus', so Venus would have remained the second planet. On the other hand, if Mercury had exploded, Venus (or rather 'Venus') would have been the first planet.

- This may look nit-picking. But there is a substantial point here, which you yourself raise in your essay, whether the other Ians are *you*, or only your *counterparts*. You can tell a similar story to the Mercury/ Venus story of different ways of understanding 'counterfactual' statements about 'Ean' (i.e. Ean might have done so-and-so, instead of doing so-and-so).

That's the first thing I wanted to say.

Now, here's one argument you can use against Ian2, if he claims that the universe he comes from is 'another possible world'. The Inflationary Universe theory might be right, who knows. Maybe there are other worlds out there, and maybe individuals from those worlds could travel by a transworld express ship and meet up. However, that would *mean* that the those worlds are actual worlds, not possible worlds. The actual world, according to the Inflationary Universe theory, contains lots of 'worlds' just as the universe on the traditional view contains lots of stars and galaxies. No big deal. The biggest problem (which is dealt with in a later unit) has to do with the idea that there might be more than one 'space'.

It is true that David Lewis, who takes the strongly realist line, talks of other worlds being in 'different spaces and times'. That is simply a logical consequence of the assumption that the actual world is *no more real* than other possible worlds. (In terms of the idea of a transworld express, the point about 'other times' does create problems however, if you think about it.)

What is the difference between other 'actual' worlds and other 'possible' worlds? This is a question of logic. The Inflationary Universe theory makes a factual claim: what it claims is that this is the way the universe is actually constituted. It would therefore be perfectly meaningful to consider a possible world where the Inflationary Universe theory is false! There might not have been an Inflationary Universe, but only this lonely cosmos. If we can meaningfully, coherently speak of a there being a possible world where the Inflationary Theory is false, that world cannot, by definition, be one of the worlds that belong to the Inflationary Universe. (NB, when I say that the Inflationary Theory might have been false I don't just mean that no-one might have believed it, but that it simply might not have been the way things are.)

However, as a matter of logic, there is a possible world corresponding to *every* logical possibility, as you admit. That is sufficient to prove that possible worlds, in the true sense, are not the same as the alternative worlds which make up the Inflationary Universe.

All the best,

Geoffrey