Take Thirst

Let us inquire first into how plausible it is to suppose that specific biochemical powers of the brain are really irrelevant to the mind. . . . [I]f you consider specific mental states and processes—being thirsty, wanting to go to the bathroom, worrying about your income tax, trying to solve math puzzles, feeling depressed, recalling the French word for “butterfly“—then it seems at least a little odd to think that the brain is so irrelevant.

Take thirst, where we actually know a little about how it works. Kidney secretions of renin synthesize a substance called angiotensin. This substance goes into the hypothalamus and triggers a series of neuron firings. As far as we know these neuron firings are a very large part of the cause of thirst. Now obviously there is more to be said, for example, about the relations of the hypothalamic responses to the rest of the brain, about other things going on in the hypothalamus, and about the possible distinctions between the feeling of thirst and the urge to drink. Let us suppose we have filled out the story with the rest of the biochemical causal account of thirst.

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Now the theses of the mind as program and the irrelevance of the brain would tell us that what matters about this story is not the specific biochemi- cal properties of the angiotensin or the hypothalamus but only the formal computer programs that the whole sequence instantiates. Well, let’s try that out as a hypothesis and see how it works. A computer can simulate the for- mal properties of the sequence of chemical and electrical phenomena in the production of thirst just as much as it can simulate the formal properties of anything else—we can simulate thirst just as we can simulate hurricanes, rainstorms, five-alarm fires, internal combustion engines, photosynthesis, lactation, or the flow of currency in a depressed economy. But no one in his right mind thinks that a computer simulation of a five-alarm fire will burn down the neighborhood, or that a computer simulation of an internal com- bustion engine will power a car or that computer simulations of lactation and photosynthesis will produce milk and sugar. To my amazement, however, I have found that a large number of people suppose that computer simulations of mental phenomena, whether at the level of brain processes or not, literally produce mental phenomena.