‘The brain is a computer’ – this claim is as central to our scientific understanding of the mind as it is baffling to anyone who hears it. We are either told that this claim is just a metaphor or that it is in fact a precise, well-understood hypothesis. But it’s neither. We have clear reasons to think that it’s literally true that the brain is a computer, yet we don’t have any clear understanding of what this means. That’s a common story in science.
To get the obvious out of the way: your brain is not made up of silicon chips and transistors. It doesn’t have separable components that are analogous to your computer’s hard drive, random-access memory (RAM) and central processing unit (CPU). But none of these things are essential to something’s being a computer. In fact, we don’t know what is essential for the brain to be a computer. Still, it is almost certainly true that it is one.
I expect that most who have heard the claim ‘the brain is a computer’ assume it is a metaphor. The mind is a computer just as the world is an oyster, love is a battlefield, and this shit is bananas (which has a metaphor inside a metaphor). Typically, metaphors are literally false. The world isn’t – as a matter of hard, humourless, scientific fact – an oyster. We don’t value metaphors because they are true; we value them, roughly, because they provide very suggestive ways of looking at things. Metaphors bring certain things to your attention (bring them ‘to light’), they encourage certain associations (‘trains of thought’), and they can help coordinate and unify people (they are ‘rallying cries’). But it is nearly impossible to ever say in a complete and literally true way what it is that someone is trying to convey with a literally false metaphor. To what, exactly, is the metaphor supposed to turn our attention? What associations is the metaphor supposed to encourage? What are we all agreeing on when we all embrace a metaphor?
If it were a metaphor to say that the brain is a computer, then we would expect the claim to be literally false. This checks out with the point that our brains aren’t organised, like PCs, into silicon-based hard drives, RAMs and CPUs. We would also expect it to be difficult to flesh out exactly what we mean when we say that the brain is a computer. The value of the claim, were it a metaphor, would have to lie in whether it suggests the right things to attend to, whether it calls to mind fruitful associations, and whether it succeeds in bringing some coordination to the cognitive sciences. Some think that the supposed metaphor succeeds on these counts, while others think it fails and has poisoned the well of cognitive-science research.
But the claim that the brain is a computer is not just a metaphor. The cognitive sciences are full of hypotheses to the effect that the brain computes such-and-such in so-and-so a way. Many of our perceptual capacities, for example, are understood in computational terms, and there aren’t any viable alternatives around. Here are a handful of widely accepted hypotheses about what the brain computes, though I will leave out the details:
- Direction of sound from interaural time difference: if a loud crash occurs directly in front of you, its soundwaves will reach your left and right ears at the same time. If it occurs to your left, its soundwaves will reach your left ear slightly before they reach your right ear. When you hear a loud crash as being to your left or your right, your auditory system is computing, according to trigonometric principles, an estimate of that crash’s direction on the basis of the difference in times between when the sound waves arrived at your right and your left ears.
- Depth from disparity (or stereopsis): most things reflect light to both your eyes. Take one of your fingers and hold it arm’s length away from you, and take another finger and hold it halfway between the farther finger and your face. Now fix your gaze on the closer finger. Your farther finger will reflect light to a different part of your left eye (relative to its centre) than it will to your right eye