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how fast they can react, and what precisely they perceive. 7 For the first time, perceptions began to be measured with scientific rigor, and surprises began to leak out. For example, it seemed obvious that your senses give you an accurate representation of the outside world—but by 1833 a German physiologist namedJohannes Peter Müller (1801–1858)had noticed something puzzling. If he shone light in the eye, put pressure on the eye, or electrically stimulated the nerves of the eye, all of these led to similar sensations of vision—that is, a sensation of
light
rather than of pressure or electricity. This suggested to him that we are not directly aware of the outside world, but instead only of the signals in the nervous system. 8 In other words, when the nervous system tells you that something is “out there”—such as a light—that is what you will believe, irrespective of how the signals get there.
    The stage had now been set for people to consider the physical brain as having a relationship with perception. In 1886, years after both Weber and Müller had died, an American namedJames McKeen Cattell published a paper entitled “The time taken up by cerebral operations.” 9 The punch line of his paper was deceptively simple: how quickly you can react to a question depends on the type of thinking you have to do. If you simply have to respond that you’ve seen a flash or a bang, you can do so quite rapidly (190 milliseconds for flashes and 160 milliseconds for bangs). But if you have to make a choice (“tell me whether you saw a red flash or a green flash”), it takes some tens of milliseconds longer. And if you have to name what you just saw (“I saw a blue flash”), it takes longer still.
    Cattell’s simple measurements drew the attention of almost no one on the planet, and yet they were the rumblings of a paradigm shift. With the dawning of the industrial age, intellectuals were thinking about
machines
. Just as people apply the computer metaphor now, themachine metaphor permeated popular thought then. By this point, the later part of the nineteenth century, advances in biology had comfortably attributed many aspects of behavior to the machinelike operations of the nervous system. Biologists knew that it took time for signals to be processed in the eyes, travel along the axons connecting them to the thalamus, then ride the nerve highways to the cortex, and finally become part of the pattern of processing throughout the brain.
    Thinking
, however, continued to be widely considered assomething different. It did not seem to arise from material processes, but instead fell under the special category of the mental (or, often, the spiritual). Cattell’s approach confronted the thinking problem head-on. By leaving the stimuli the same but changing the task (
now make such-and-such type of decision
), he could measure how much longer it took for the decision to get made. That is, he could measure
thinking time
, and he proposed this as a straightforward way to establish a correspondence between the brain and the mind. He wrote that this sort of simple experiment brings “the strongest testimony we have to the complete parallelism of physical and mental phenomena; there is scarcely any doubt but that our determinations measure at once the rate ofchange in the brain and of change in consciousness.” 10
    Within the nineteenth-century zeitgeist, the finding that thinking takes time stressed the pillars of the thinking-is-immaterial paradigm. It indicated that thinking, like other aspects of behavior, was not tremendous magic—but instead had a mechanical basis.

     
    Could thinking be equated with the processing done by the nervous system? Could the mind be like a machine? Few people paid meaningful attention to this nascent idea; instead, most continued to intuit that their mental operations appeared immediately at their behest. But for one person, this simple idea changed everything.
ME, MYSELF, AND THE ICEBERG
     
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