Do Flying Donkeys Exist? David Lewis Says Yes

‘Who is David Lewis?’

‘He is one of the greatest philosophers of the century.’

‘Goodness. And what does he believe?’

‘That all possible worlds actually exist.’

‘But what can that mean, it makes no sense: do you believe it?’

‘No.’

This is a rather surreal conversation, for how can you claim that somebody is the greatest philosopher in the world and then immediately add that his main thesis is not even credible? And yet I have had this conversation, almost verbatim, a surprising number of times with a surprising number of eminent philosophers of various nationalities. In the somewhat rarefied atmosphere of analytic philosophy, the American philosopher with strong links to Australia, who died just over a decade ago, is today recognized by many of his colleagues as one of the greatest, if not the greatest, of all contemporary philosophers – even if his most well-known thesis, the concrete existence of many worlds, leaves many scratching their heads.

Lewis was an appealing character. In his lectures he was prone to talk about science-fiction movies and time travel. His philosophical articles are crowded with flying donkeys, cats that lose all their fur on sofas, and so on. He had a bushy beard, and an estranged air about him. He exuded something unconventional and slightly unhinged. He loved Australia, where he spent many months each year. He wrote dozens of articles on the widest range of subjects, all of interest to analytical philosophy, and a few books, the most notable of which is On the Plurality of Worlds. As its title suggests, it focuses his main thesis: that all possible worlds exist. Including those in which donkeys fly.

But wait a minute, you will rightly say, there are no flying donkeys in this world. And Lewis agrees, there are no donkeys that can fly in this world. But in other worlds there are. There are many worlds in which donkeys fly. All of these worlds are really there. They exist. We do not see flying donkeys because we live in a world where they do not fly. Just as from my window I can see the sea of Marseille and not the Colosseum, because I am in Marseille and not Rome. Not because Marseille exists and Rome does not. Just as Rome exists even when I am not there, so other worlds exist even though we are not in them. And which of these other worlds exists, exactly? They all do, replies Lewis, behind a disarming smile.

Slightly perplexed by hearing about these ideas, last year I decided to try reading Lewis, even though I am not a philosopher and don’t have all the necessary conceptual tools at my disposal.

The first thing I read was an article about time travel. Given that my own subject is physics, and that I am particularly interested in the nature of space and time, I imagined that I would be in quite a strong position to tackle philosophy in a field with which I am familiar. Besides, I’ve always found that what I usually read on the impossibility of time travel is confused and messy, and I expected more of the same. To tell the truth, I began reading the article fully expecting to catch the supposedly great philosopher making some basic, elementary mistake. Instead I soon found myself open-mouthed. Lewis discusses the possibility of travelling in time with complete clarity. His article is utterly, unequivocally clear. He has brought perfect order to the question. All of the foolish assertions about how, if we went back in time, we could end up by killing our grandfathers are swept aside with lucid simplicity. I began to see why so many people are dazzled by David Lewis.

And so I immersed myself in a collection of his articles. A few of the more technically philosophical ones bored me, frankly, and I failed to understand them. But many struck me as exceptionally brilliant. What exactly, asks Lewis, is a thing, an entity? A cat that is lying on a sofa, for instance. Does it include the hair that has left the cat and adhered to the sofa? Where exactly does the cat end and its context begin? And so on and so forth, zigzagging between technical problems of modal logic and those questions we used to discuss as teenagers and never found answers for. For every question, Lewis has a convincing answer, even though he always presents them with a smile on his lips. He finds solutions precisely where there seemed to be none, with dazzling intelligence.

Armed with this experience, I felt ready to tackle his major work, The Plurality of Worlds. Now let him try to convince me, I found myself muttering, that flying donkeys exist …

I have to admit defeat, to confess that he really did manage to persuade me. I will not even attempt to summarize how. I am not Lewis. If you are at all intrigued, do read his book. I still have a few doubts. I still ask myself, for instance, whether Lewis does not merely change the meaning of certain words, using ‘to exist’ where others would use ‘to be possible’, and calling ‘of this world’ what others would call ‘existent’. But he has undoubtedly changed my ideas about what ‘to exist’ means. He has helped to free me, I think, from prejudices that are still attached to this notoriously slippery verb. Does a puppet exist whose nose grows when he tells lies? Yes, of course it does, it’s Pinocchio. So Pinocchio exists? No, he doesn’t exist! But you just said that he exists …

At the very least, Lewis has completely convinced me that to speak of possible worlds as if they were real is a very effective tool for achieving clarity in all questions involving modality, which is to say those arguments pertaining to possibility or necessity.

Thanks to Lewis and his colleagues, analytical philosophy has returned to engaging weightily with metaphysics – terrain that for so long it had kept a safe distance from. The lesson of logical positivism, which had insisted upon affirming that we should speak only of those things that could be defined in a sufficiently clear manner – and above all the one provided by Wittgenstein, who showed how many apparently profound problems are nothing but the result of clumsy and inexact uses of language – had left a deep mark on this vast area of philosophy, with the result that questions revolving around existence and non-existence had become habitually regarded with scepticism in these quarters. Even today the word ‘metaphysics’ has the capacity to prompt a collective raising of eyebrows in some philosophy departments around the world. Lewis himself is sometimes subject to this kind of suspicion. Yet there is undoubtedly a part of analytical philosophy that has found a way, with the clarity of thought that characterizes it, and with its own tools, of returning to dealing with questions such as what exists or does not exist. Lewis has contributed significantly to bringing metaphysics back to the centre of discussion.