TORTOISE: I am glad that for once we seem to be in agreement in our conclusions. I see that the hour is growing late; dusk is approaching—just the time when all my forces seem to gather, and I feel quite energetic. I know you must have been disappointed by the “no-show” of your friend; how’s about a little footrace back to the fifth century B.C.?
ACHILLES: What a capital idea! But just to be fair, I’ll give you a head start of, oh, three centuries, since I’m so fleet of foot.
TORTOISE: You’re a mite cocky, Achilles.... You may not find it so easy to catch up with an Energetic Tortoise.
ACHILLES: Only a fool would bet on a slow-footed Tortoise, racing against me. Last one to Zeno’s house is a monkey’s uncle!
Reflections
“Well, all these fantasies have been fun, but they can’t really tell us anything. They’re just so much science fiction. If you want to learn the truth—the hard facts—about something, you have to turn to real science, which so far has had little to tell us about the ultimate nature of the mind.” This response conjures up a familiar but impoverished vision of science as a collection of precise mathematical formulae, meticulous experiments, and vast catalogues of species and genera, ingredients and recipes. This is the picture of science as strictly a data-gathering enterprise in which imagination is tightly reined in by incessant demands for proof. Even some scientists have this vision of their profession, and are deeply suspicious of their more playful colleagues, however eminent. Perhaps some symphony orchestra players view their business as nothing but precise noise-making produced under conditions of militaristic discipline. If so, think what they are missing.
In fact, of course, science is an unparalleled playground of the imagination, populated by unlikely characters with wonderful names (messenger RNA, black holes, quarks) and capable of performing the most amazing deeds: sub-atomic whirling dervishes that can be in several places—everywhere and nowhere—at the same time; molecular hoop-snakes biting their own tails; self-copying spiral staircases bearing coded instructions; miniature keys searching for the locks in which they fit, on floating odysseys in a trillion synaptic gulfs. So why not brain-book immortality, dream-writing machines, symbols that understand themselves, and fraternal homunculi without arms, legs, or heads, sometimes blindly following orders like the sorcerer’s broom, sometimes feuding and conniving, sometimes cooperating? After all, some of the most fantastic ideas presented in this book—Wheeler’s solitary electron weaving the universe, for example, or Everett’s many-worlds interpretation of quantum mechanics, or Dawkins’ suggestion that we are survival machines for our genes—have been proposed in complete seriousness by eminent scientists. Should we take such extravagant ideas seriously? We should certainly try, for how else will we ever learn whether these are the conceptual giant steps we need to escape from the most obscure riddles of the self and consciousness? Coming to understand the mind will probably require new ways of thinking that are at least as outrageous—at first—as Copernicus’s shocking suggestion that the Earth goes around the Sun, or Einstein’s bizarre claim that space itself could be curved. Science advances haltingly, bumping against the boundaries of the unthinkable: the things declared impossible because they are currently unimaginable. It is at the speculative frontier of thought experiment and fantasy that these boundaries get adjusted.
Thought experiments can be systematic, and often their implications can be rigorously deduced. Consider Galileo’s crystal-clear reductio ad absurdum of the hypothesis that heavier objects fall faster than lighter objects. He asks us to imagine taking a heavy object, A, and a light object, B, and tying them together with a string or chain before dropping them off a tower. By hypothesis, B falls slower, and hence should act as a drag on A; thus A tied to B should fall slower than A by itself. But A tied to B is itself a new object, C, which is heavier than A, and hence, by hypothesis, C should fall faster than A by itself. A tied to B cannot at the same time fall faster and slower than A by itself (a contradiction or absurdity), so the hypothesis must be false.
On other occasions thought experiments, however systematically developed, are intended merely to illustrate and enliven difficult ideas. And sometimes the boundaries between proof, persuasion, and pedagogy cannot be drawn. In this book there are a variety of thought experiments designed to explore the implications of the hypothesis that materialism is true: the mind or self is not another (non-physical) thing, in miraculous interaction with the brain, but somehow a natural and explainable product of the brain’s organization and operation. “The Story of a Brain” presents a thought experiment that is meant, like Galileo’s, to be a reductio ad absurdum of its main premise—in this case, materialism in the guise of “the neural theory of experience.” “Prelude, Ant Fugue,” “Where Am I?” and “A Conversation with Einstein’s Brain,” on the other hand, are designed to support materialism by helping thinkers over obstacles that have traditionally stood in the way of comprehending it. In particular, these thought experiments are designed to provide a plausible substitute for the otherwise compelling idea of the self as a sort of mysterious, indivisible pearl of mind-stuff. “Minds, Brains, and Programs” is intended to refute one version of materialism (roughly, the version we defend), while leaving some underdescribed and unexplored materialistic alternatives untouched.
In each of these thought experiments there is a narrative problem of scale: how to get the reader’s imagination to glide over a few billion details and see the woods and not just the trees. “The Story of a Brain” is silent about the staggering complexity of the devices to which the imagined brain parts would have to be attached. In “Where Am I?” the virtual impossibility of using radio links to preserve the connectivity in hundreds of thousands of nerves is conveniently ignored, and the even less likely feat of making a computer duplicate of a human brain that could operate synchronously is presented as nothing more than a fancy bit of technology. “Minds, Brains, and Programs” invites us to imagine a person hand simulating a language-processing program which, if it were realistic, would be so huge that no person could perform the steps for a single interchange in less than a lifetime, but we are cajoled into imagining the system engaging in Chinese conversations occurring in a normal time scale. The problem of scale is faced directly in “A Conversation with Einstein’s Brain,” where we are asked to tolerate a book with a hundred billion pages we can flip through fast enough to extract a few conversational gems from the posthumous Prof. Einstein.
Each setting of the dials on our intuition pump yields a slightly different narrative, with different problems receding into the background and different morals drawn. Which version or versions should be trusted is a matter to settle by examining them carefully, to see which features of the narrative are doing the work. If the oversimplifications are the source of the intuitions, rather than just devices for suppressing irrelevant complications, we should mistrust the conclusions we are invited to draw. These are matters of delicate judgment, so it is no wonder that a generalized and quite justified suspicion surrounds such exercises of imagination and speculation.