The art of the possible

The art of the possible
Everyday Philosophy


What’s the proper role of common sense in philosophy? In good thinking? The answer isn’t as straightforward as we might hope.

I’ll begin as I often do by asking “what do we mean by common sense?” This time I’ll try to characterise it by examining its absence. We’re all familiar with cases where philosophers’ theories seem to venture out beyond the realm of common sense. My favourite example is David Lewis’s theory about possible worlds.

Talk of ‘possible worlds’ has a variety of purposes: one of them being to discriminate between what is truly impossible and what is merely very unlikely.”

Philosophers often like to talk about ‘possible worlds’. At its best, this is a useful shorthand for talking about different ways that things in the world could have been if certain events or happenings had gone differently. “In some possible world, Hillary Clinton is the President of the United States”.

Talk of ‘possible worlds’ has a variety of purposes: one of them being to discriminate between what is truly impossible and what is merely very unlikely. There is nothing impossible about the idea of a talking donkey, and so in some possible world there are talking donkeys. Indeed, given that there is an infinite array of things that are not impossible, there is some possible world in which the only living things are talking donkeys (presumably getting around under the power of photosynthesis or similar). All donkeys, all the time.

Now you could dispute all that (is such a world really even possible?) but there’s nothing anti-common-sense about talking in terms of possible worlds, so long as you’re clearly just using it as a way of speaking about possibilities, counterfactuals, ways things might have been.

David Lewis did something very different: he argued that possible worlds are quite literally real. All of them. What we mean when we say that something is possible is that it is really the case in some other world. There really is a world that comprised only of donkeys, all the time.

Now, this just seems mad. It clashes so dramatically with common sense that it strikes us as something we can rule out immediately. It’s this sort of all-donkeys-all-the-time stuff that often gives philosophy a bad name: helping to craft the stereotype of philosophers as people obsessed with bizarre, abstract theories; cloud-castles that have no relevance to reality.

Here’s the thing: I have never read David Lewis’ writings on possible worlds. I have never assessed his arguments for his dramatic conclusion. But I know some of his other work, and I am thoroughly convinced that he was dramatically smarter than I am. I think he had a genius-level intellect. And I’m sure his arguments for being a realist about possible worlds were sophisticated and well-thought-out.

What’s more, sometimes crazily counterintuitive ideas that blast common sense into smithereens are just correct. Plenty of scientific theories later proven correct were extremely anti-common-sense (heliocentrism!). It’s hard to overstate just how counterintuitive the complete abolition of slavery as a societal institution would have been in, say, Ancient Greece or pre-Christian Rome – there are no surviving records of anyone from those societies proposing to get rid of slavery entirely.

It’s easy for the unquestioned assumptions of a particular era or tradition of thought to get coded as forever-unchanging common sense, necessary to quash any ideas that seem too outlandish or radical. At my English university, I encountered this kind of tyranny of common sense in discussions about religion. There was often a sense that belief in God, miracles, or the resurrection were just unserious – that these things were too weird to be believed by anyone who had any common sense, however good the arguments might be. But in another age, it would have been materialism that seemed absurd and unserious. It’s undoubtedly the case that common sense can be an unreliable guide.

Nonetheless, I think we can’t really do without some idea of common sense. This is partly just because of time. None of us has the time to consider in detail every theory or idea that has intelligent backers and sophisticated arguments behind it. We would never get anywhere in the search for truth if we didn’t have some way of ruling out some theories as just too far out. If you don’t have some kind of account of common sense, you’ll risk getting taken in by elegant, well-defended nonsense – or at least wasting swathes of time trying to prove it false.

The truth is that the precise balance between radical philosophical questioning and common sense isn’t something that can be decided in advance. You need to stay attached enough to common sense to avoid losing touch with reality: and you need to be willing to question common sense for precisely the same reason. I don’t think there’s a real world that is all donkeys, all the time. But I’ve been wrong before – and so has common sense.