The final installment of our special series is given by Timothy Williamson

One idea that has mattered to me over the last decade is that philosophical knowledge grows through the extraordinarily relentless, systematic application of quite ordinary human cognitive skills. A challenge to this idea is that although ordinary cognitive skills are well adapted for providing knowledge that something is so, it is much less obvious how they are to provide knowledge that something must or could be so, which is what philosophers are usually interested in. I can see that my sofa is red, but how could I see that it could have been green, or that it could not have been red without being coloured? Philosophers have come up with all sorts of strange and implausible theories about how we know such facts about possibilities and impossibilities: that we do it by a mysterious faculty of rational intuition, or by applying the grammar of the English language, or that there are really no such facts to be known.

What I have tried to show is that our ability to know facts about what could or could not have been the case is just a natural extension of our ordinary ability to know everyday facts about what would have been the case if things had been different in various ways. For example, I know that if I had flicked a certain switch just now, my light would have gone off. The conditional is counterfactual; in fact I did not flick the switch. The knowledge is not particularly philosophical. It depends on my knowledge that the light is on and that this is the light switch. We need knowledge of such counterfactual conditionals in order to learn from our mistakes. You do something, it turns out badly, and you realise that if you had acted differently, the outcome would have been better.

The imagination plays a central role in enabling us to gain such knowledge. You imagine what train of events would have occurred if you had acted differently in a certain way. Of course, we can also imagine lots of wild, arbitrary possibilities. That function of the imagination has practical value too, in alerting us to dangers and opportunities we might otherwise have missed. But in imagining what would have happened if such-and-such had happened, we are tightly constrained by what we already know. We use many of the same cognitive skills offline in the imagination that we use online when we engage with the external world in perception and action. Imagining what would have happened if you had called someone yesterday is not so different from predicting what will happen if you call her tomorrow. Most of our knowledge of cause and effect depends on exploring counterfactual scenarios. Although physical laws and calculation sometimes help us do so, we must still think imaginatively in selecting appropriately simple mathematical models of complex situations. Imagination can provide knowledge, not just random guesses, because it is reality-directed.

I suggest that in philosophical and scientific thought experiments, we use our imaginations in just such cognitively constrained ways, to gain knowledge of what would have been in various counterfactual circumstances, and thereby to gain knowledge about which things are possible and which impossible. The necessary is that which would have been however things had been; the possible is that whose contradictory is not necessary. Our ability to use imagination to explore the limits of the possible is a fortunate by-product of our evolutionarily adaptive ability to use it to explore much more practical matters.

Although philosophers have discussed the relation between possibility and imaginability (or conceivability), they have usually done so while neglecting the central cognitive significance of the imagination in our lives, and so viewed it from a perspective that made its reality-directedness impossible to understand. The result was to give undeserved credibility to sceptical or obscurantist theories of thought about the possible.

The background to my epistemology of possibility is a more general theme: anti-exceptionalism about philosophy. A professional deformation of those engaged in any discipline is to present it as more exceptional than it really is; philosophers are no exception. Of course, the branches of human inquiry differ from each other in topic and method: consider mathematics, physics, biology, psychology, economics, history, linguistics… Philosophy is a legitimate part of that diversity. Any discipline which neglects its special skills in slavish imitation of another is failing to make its distinctive contribution to the growth of human knowledge.

But when philosophers claim that philosophical understanding is more special than that, nothing as ordinary as a branch of human knowledge, they raise the suspicion that philosophy is a scam. If you don’t know why something is so, you don’t understand why it is so either. We can’t properly appreciate how philosophy differs from other disciplines until we appreciate how it is the same.

Further reading
The Philosophy of Philosophy, Timothy Williamson (Blackwell, 2007)

Timothy Williamson is Wykeham Professor of Logic at the University of Oxford

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