To: Patricia A.
From: Geoffrey Klempner
Subject: Are possible worlds really real?
Date: 18th August 2008 12:51

Dear Patricia,

Thank you for your email of 5 August, with your first essay for Possible World Machine, in response to the question, 'Are possible worlds really 'real'?'

How does one tackle a question like this? The first thing one needs to do is to explain what the claim amounts to. As you state in your opening paragraph, the claim that possible worlds are 'really real' amounts to the claim that corresponding to each possible way the actual world might have been, there exists an actual world, in its own space and time which are discontinuous from the space and time of the actual world.

It follows that the quality of 'actuality' is reducible to perspective. Any possible world is 'actual' to the inhabitants of that world; as our world is 'actual' to us. It also follows that the world we call 'actual' is no more 'real' than any other possible world.

This is, as you state following the OED definition, a claim that each possible world possesses 'existence in fact and not merely in appearance, thought, or language'.

In order to answer the question, there are various lines that need to be considered: first, what are the arguments which have been put forward for the claim in question? are they valid, or can you see a loophole? Is the claim in question coherent, does it make sense? (if it doesn't make sense, then, logically, there can't be a valid argument). Is there an alternative way, an alternative theory, to account for the considerations adduced in favour of the claim in question?

You don't mention the argument for the real existence of possible worlds. The argument goes like this:

1. Whenever we hold that a statement P is true, anything whose existence is implied by the truth of P must exist.

2. In order to account for the truth of 'counterfactual' statements, of the form, 'If it had been the case that P then it would have been the case that Q', it is necessary to recognize the real existence of possible worlds. Statements about the actual world are made true by facts about the actual world. Statements about other possible worlds are made true by facts about other possible worlds.

Although you don't say what you think is wrong with this argument, you offer an alternative account of possible worlds in terms of the human ability to conceive of the 'fictive or the imaginary' which implies an objection to the above argument.

The objection would go like this. When I make a statement about how things might be in another possible world, I am not describing a fact but rather mentally manipulating things that I know about the actual world.

Consider, for example, 'If gravity had been weaker human beings would have been able to jump higher.'

On the first theory, this statement is true by virtue of the fact that the possible worlds which are most similar to the actual world where gravity is less (i.e. because the earth is smaller) are worlds where human beings are able to jump higher. (I am following the analysis of counterfactual statements given by David Lewis, foremost defender of the 'realist' view of possible worlds, in his book 'Counterfactuals'.)

The concept of 'similarity' is crucial here. In describing the relevant possible world(s), we are assuming human beings as they are and merely changing the size of the earth, because this is, or seems, more 'similar' to the actual world than a scenario where human beings have evolved with less strength because gravity is weaker and therefore can only jump as high as we can jump. This is a point on which Lewis can be challenged: it seems there is no way to resolve this kind of argument except by appeal to 'intuitions' concerning what we understand by 'similar'.

On the second, fictive, theory, the statement is true by virtue of, what exactly? What kind of fact are we stating? Or is it not a fact?

Before you give in to the temptation to say that we are not 'stating a fact', remember that we make counterfactual statements all the time, and in many circumstances important consequences follow from our decision regarding their truth or falsity. Consider, for example, the use of counterfactual statements in a court of law where the prosecution and defence are arguing over the motives of the accused. Or the use of counterfactual statements in the context of performing a scientific experiment.

I hope that you can see that there is a problem here. There is no denying that human beings have a power of imagination, and that when we make counterfactual statements, we consult our imaginations or represent possible states of affairs to ourselves in our imaginations. However, this is only the subjective aspect. After all, we also use our imaginations when we make ordinary factual statements too.

By contrast, the objective aspect is concerned with truth and falsity, existence or non-existence. If some counterfactual statements are true, as they seem to be, we need an explanation of the kinds of 'fact' by virtue of which they are true. I am not saying that I agree with David Lewises theory. In fact, my sympathies are more in line with the view that the notion of 'possibility' is 'sui generis' and incapable of being analysed, as Lewis attempts to do, in terms of the 'real' existence of possible worlds. But I am also aware that it is very difficult to come up with an alternative explanation for the 'truth conditions' of counterfactual statements.

All the best,

Geoffrey