Yes, Dennett has recanted. Alright, he hasn’t finally acknowledged that Jesus is his Lord and Saviour. He hasn’t declared that qualia are the real essence of consciousness after all. But his new book From Bacteria to Bach and Back does include a surprising change of heart.
The book is big and complex: to be honest it’s a bit of a ragbag (and a bit of a curate’s egg). I’ll give it the fuller review it deserves another time, but it seems to be worth addressing this interesting point separately. The recantation arises from a point on which Dennett has changed his mind once before. This is the question of homunculi. Homunculi are ‘little people’ and the term is traditionally used to criticise certain kinds of explanation, the kind that assume some module in the brain is just able to do everything a whole person could do. Those modules are effectively ‘little people in your head’, and they require just as much explanation as your brain did in the first place. At some stage many years ago, Dennett decided that homunculi were alright after all, on certain conditions. The way he thought it could work was an hierarchy of ever stupider homunculi. Your eyes deliver a picture to the visual homunculus, who sees it for you; but we don’t stop there; he delivers it to a whole group of further colleagues; line-recognising homunculi, colour-recognising homunculi, and so on. Somewhere down the line we get to an homunculus whose only job is to say whether a spot is white or not-white. At that point the function is fully computable and our explanation can be cashed out in entirely non-personal, non-mysterious, mechanical terms. So far so good, though we might argue that Dennett’s ever stupider routines are not actually homunculi in the correct sense of being complete people; they’re more like ‘black boxes’, perhaps, a stage of a process you can’t explain yet, but plan to analyse further.
Be that as it may, he now regrets taking that line. The reason is that he no longer believes that neurons work like computers! This means that even at the bottom level the reduction to pure computation doesn’t quite work. The reason for this remarkable change of heart is that Terrence Deacon and others have convinced Dennett that the nature of neurons as entities with metabolism and a lifecycle is actually relevant to the way they work. The fact that neurons, at some level, have needs and aims of their own may ground a kind of ‘nano-intentionality’ that provides a basis for human cognition.
The implications are large; if this is right then surely, computation alone cannot give rise to consciousness! You need metabolism and perhaps other stuff. That Dennett should be signing up to this is remarkable, and of course he has a get-out. This is that we could still get computer consciousness by simulating an entire brain and reproducing every quirk of every neuron. For now that is well beyond our reach – and it may always be, though Dennett speaks with misplaced optimism about Blue Brain and other projects. In fact I don’t think the get-out works even on a theoretical level; simulations always leave out some aspect of the thing simulated, and if this biological view is sound, we can never be sure that we haven’t left out something important.
But even if we allow the get-out to stand this is a startling change, and I’ve been surprised to see that no review of the book I’ve seen even acknowledges it. Does Dennett himself even appreciate quite how large the implications are? It doesn’t really look as if he does. I would guess he thinks of the change as merely taking him a bit closer to, say, the evolution-based perspective of Ruth Millikan, not at all an uncongenial direction for him. I think, however, that he’s got more work to do. He says:
The brain is certainly not a digital computer running binary code, but it is still a kind of computer…
Later on, however, he rehashes the absurd but surely digitally-computational view he put forward in Consciousness Explained:
You can simulate a virtual serial machine on a parallel architecture, and that’s what the brain does… and virtual parallel machines can be implemented on serial machines…
This looks pretty hopeless in itself, by the way. You can do those things if you don’t mind doing something really egregiously futile. You want to ‘simulate’ a serial machine on a parallel architecture? Just don’t use more than one of its processors. The fact is, parallel and serial computing do exactly the same job, run the same algorithms, and deliver the same results. Parallel processing by computers is just a practical engineering tactic, of no philosophical interest whatever. When people talk about the brain doing parallel processing they are talking about a completely different and much vaguer idea and often confusing themselves in the process. Why on earth does Dennett think the brain is simulating serial processing on a parallel architecture, a textbook example of pointlessness?
It is true that the brain’s architecture is massively parallel… but many of the brain’s most spectacular activities are (roughly) serial, in the so-called stream of consciousness, in which ideas, or concepts or thoughts float by not quite in single file, but through a Von Neumann bottleneck of sorts…
It seems that Dennett supposes that only serial processing can deliver a serially coherent stream of consciousness, but that is just untrue. On display here too is Dennett’s bad habit of using ‘Von Neumann’ as a synonym for ‘serial’. As I understand it the term “Von Neumann Architecture” actually relates to a long-gone rivalry between very early computer designs. Historically the Von Neumann design used the same storage for programs and data, while the more tidy-minded Harvard Architecture provided separate storage. The competition was resolved in Von Neumann’s favour long ago and is as dead as a doornail. It simply has no relevance to the human brain: does the brain have a Von Neumann or Harvard architecture? The only tenable answer is ‘no’.
Anyway, whatever you may think of that, if Dennett now says the brain is not a digital computer, he just cannot go on saying it has a Von Neumann architecture or simulates a serial processor. Simple consistency requires him to drop all that now – and a good thing too. Dennett has to find a way of explaining the stream of consciousness that doesn’t rely on concepts from digital computing. If he’s up for it, we might get something really interesting – but retreat to the comfort zone must look awfully appealing at this stage. There is, of course, nothing shameful in changing your mind; if only he can work through the implications a bit more thoroughly, Dennett will deserve a lot of credit for doing so.
More another time.