 brain that make it distinctively computational. It gets worse: the features of single neurons or networks of neurons that make them distinctively computational might differ from the more high-level features of our mental capacities – our abilities to perceive the direction of a sound source or the depth of a surface – that make those capacities distinctively computational.
There is just no consensus on what it is to be a computation or for a piece of nature to perform a computation. Neither question might have a general answer. The common claim that the brain is a computer is not, at the moment, a concrete, precise, well-understood scientific hypothesis.
Still, the claim is almost certainly true.
Earlier I said that the cognitive sciences are full of concrete, well-understood hypotheses that the brain computes such-and-such in so-and-so way. Now I am saying that the claim that the brain computes is not a concrete, well-understood hypothesis. Worse, I am still insisting that the claim is almost certainly true. Bananas?
The concrete, well-understood hypotheses are the specific hypotheses that the brain performs specific computations. Many of these hypotheses have lots of evidence. Each of these specific hypotheses entails that the general claim that the brain computes, full stop, must be true. (Note: they do not imply that the brain only computes! It certainly does other things.) The thing is, none of this means that the general claim that the brain computes is itself a concrete and well-understood hypothesis. The hypothesis that the brain computes X can yield testable predictions and insightful explanations. The claim that the brain computes, full stop, doesn’t on its own do much predicting or explaining at all – or at least, I suspect, not nearly so much as is advertised.
Many research articles begin, in the introduction, with: ‘We are interested in whether the brain computes X,’ and then: ‘If it does compute X, then upon conducting such-and-such experiment we should see this-or-that result.’ And while scientists might disagree about whether the results are robust or whether the controls are adequate, and they might suggest additional experiments, there is rarely fundamental disagreement about what sorts of experiments are relevant to the hypothesis. It’s not so with: ‘We are interested in whether the brain computes.’ This naturally suggests… what? There’s not been wide agreement about what experiments one should run to test this general hypothesis or even about what phenomena this general hypothesis is supposed to explain. While the statement ‘the-brain-computes-X’, treated as a chunk, might be a perfectly good, well-understood scientific hypothesis, it will take a lot of work to say exactly what the crucial part of the sentence ‘The brain computes…’ says all by itself.
The claim that the brain is a computer is not merely a metaphor – but it is not quite a concrete hypothesis. It is a theoretical framework that guides fruitful research. The claim offers to us the general formula ‘The brain computes X’ as a key to understanding certain aspects of our astonishing mental capacities. By filling in the X, we get mathematically precise, testable and, in a number of cases, highly supported hypotheses about how certain mental capacities work. So, the defining claim of the theoretical framework, that the brain computes, is almost certainly true.
Though we are in a position to say that it is likely true that the brain is a computer, we do not yet have any settled idea of what it means for the brain to be a computer. Like a metaphor, it really is unclear what the literal message of the claim is. But, unlike a metaphor, the claim is intended to be a true one, and we do seek to uncover its literal message.
Even if you agree to the literal claim that the brain computes since, after all, our best theories hold that it computes X, Y and Z, you might be unsure or disagree about what it is for something to be a computer. In fact, it looks like a lot of people disagree about the meaning of something that they all agree is true. The point