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Imagine that your brain starts to deteriorate in such a way that you are slowly going blind. Imagine that the desperate doctors, anxious to alleviate your condition, try any method to restore your vision. As a last resort, they try plugging silicon chips into your visual cortex. Imagine that to your amazement and theirs, it turns out that the silicon chips restore your vision to its normal state. Now imagine further that your brain, depressingly, continues to deteriorate and the doctors continue to implant more silicon chips. You can see where the thought experiment is going already: in the end, we imagine that your brain is entirely replaced by silicon chips; that as you shake your head, you can hear the chips rattling around inside your skull. In such a situation there would be various possibilities. One logical possibility, not to be excluded on any a priori grounds alone, is surely this: you continue to have all sorts of thoughts, experiences, memories, etc., that you had previously; the sequence of your mental life remains unaffected. In this case, we are imagining that the silicon chips have the power not only to duplicate your input-output functions, but also to duplicate the mental phenomena, conscious and otherwise, that are normally responsible for your input-output functions.
I hasten to add that I don’t for a moment think that such a thing is even remotely empirically possible. I think it is empirically absurd to suppose that we could duplicate the causal powers of neurons entirely in silicon. But that is an empirical claim on my part. It is not something that we could establish a priori. So the thought experiment remains valid as a statement of logical or conceptual possibility.
But now let us imagine some variations on the thought experiment. A second possibility, also not to be excluded on any a priori grounds, is this: as the silicon is progressively implanted into your dwindling brain, you find that the area of your conscious experience is shrinking, but that this shows no effect on your external behavior. You find, to your total amazement, that you are indeed losing control of your external behavior. You find, for example, that when the doctors test your vision, your hear them say, “We are holding up a red object in front of you; please tell us what you see.” You want to cry out, “I can’t see anything. I’m going totally blind.” But you hear your voice saying in a way that is completely out of your control, “I see a red object in front of me.” If we carry this thought experiment out to the limit, we get a much more depressing result than last time. We imagine that your conscious experience slowly shrinks to nothing, while your externally observable behavior remains the same.
… To those who are puzzled how such a thing is possible, let us simply remind them: As far as we know, the basis of consciousness is in certain specific regions of the brain, such as, perhaps, the reticular formation. And we may suppose in this case that these regions are gradually deteriorating to the point where there is no consciousness in the system. But we further suppose that the silicon chips are able to duplicate the input-output functions of the whole central nervous system, even though there is no consciousness left in the remnants of the system.
Now consider a third variation. In this case, we imagine that the progressive implantation of the silicon chips produces no change in your mental life, but you are progressively more and more unable to put your thoughts, feelings, and intentions into action. In this case, we imagine that your thoughts, feelings, experiences, memories, etc., remain intact, but your observable external behavior slowly reduces to total paralysis. Eventually you suffer from total paralysis, even though your mental life is unchanged. So in this case, you might hear the doctors saying,
The silicon chips are able to maintain heartbeat, respiration, and other vital processes, but the patient is obviously brain dead. We might as well unplug the system, because the patient has no mental life at all.
Now in this case, you would know that they are totally mistaken. That is, you want to shout out,
No, I’m still conscious! I perceive everything going on around me. It’s just that I can’t make any physical movement. I’ve become totally paralyzed.
The point of these three variations on the thought experiment is to illustrate the causal relationships between brain processes, mental processes, and external observable behavior.
… What is the philosophical significance of these three thought experiments? It seems to me there is a number of lessons to be learned. The most important is that they illustrate something about the relationship between mind and behavior. What exactly is the importance of behavior for the concept of mind? Ontologically speaking, behavior, functional role, and causal relations are irrelevant to the existence of conscious mental phenomena. Epistemically, we do learn about other people’s conscious mental states in part from their behavior. Causally, consciousness serves to mediate the causal relations between input stimuli and output behavior; and from an evolutionary point of view, the conscious mind functions causally to control behavior. But ontologically speaking, the phenomena in question can exist completely and have all of their essential properties independent of any behavioral output.
— John Searle,
The Rediscovery of the Mind,
Chapter 3 – Silicon Brains, Conscious Robots, and Other Minds
Filed under: natural philosophy
