CTM would not exist without the extraordinary mathematician Alan Turing, the originator of the modern computer. In 1935–36, Turing worked on answering a mathematical question posed by David Hilbert that had long been unsolved, and in order to answer it he invented an imaginary basic computing machine, which stored information and executed a finite number of operations with the information it had stored. The information in this machine was fed to it on a tape marked with discrete symbols, either a 0 or a 1. In his biography of Turing, Andrew Hodges beautifully explicates for nonmathematicians the importance of Turing’s machine. Hodges refers to a book Turing loved as a child called Natural Wonders Every Child Should Know, in which the brain is described as “a machine, a telephone exchange or an office system.”213 This was commonplace. The English physician William Harvey (1578–1657) used the metaphor of a hydraulic system for the heart and blood circulation to great effect. Henri Bergson used the telephone switchboard as a brain metaphor. Freud called upon the telephone receiver as an analogy for the analyst.
Hodges continues, “What he [Turing] had done was to combine such a naive mechanistic picture of the mind with the precise logic of pure mathematics. His machines—soon to be called Turing machines—offered a bridge, a connection between abstract symbols, and the physical world.”214 Just around the same time, Alonzo Church also answered Hilbert’s question, but by another route altogether. What is now called the Church-Turing thesis maintains that the Turing machine can solve any computable calculation if it has enough tape and enough time. Turing published “On Computable Numbers” in 1936, and although he was disappointed in the response to it at the time, it would change the scientific landscape, including the idea of mind in psychology. Turing’s ambitions went far beyond making useful machines. His ambition was to build a brain. He wrote, “We may hope that machines will eventually compete with men in all purely intellectual fields.”215 He wanted to invent a machine that would think for itself, one that would not depend on a programmer.
In “Intelligent Machinery,” a text published after his death, Turing noted that language development in children was not due to an innate English- or French-speaking brain area and that a person’s “linguistic [brain] parts” develop through “different training.” Turing concluded, “There are large parts of the brain, chiefly in the cortex, whose function is largely indeterminate.” In children, this flexibility is much greater than in the adult. It all depends on learning—“on the training in childhood.” Turing winds up articulating the fundamental insight of behaviorism, albeit in mechanical terms: “All of this suggests that the cortex of the infant is an unorganized machine, which can be organized by suitable interfering training. This organizing might result in the modification of the machine into a universal machine or something like it.”216 Turing’s knowledge of biology was not extensive. His knowledge of mathematics was, and it rested on a fundamental assumption that certain mental operations are computable and therefore can be computed on a universal machine.
Computable operations are procedures that move forward according to a set of logical rules, step-by-step, without missing a beat—an algorithm. As Hobbes argued, one step determines the next. Because Turing’s machine is capable of imitating such procedures, it can imitate rational processes and, because a computer program is entirely legible, it follows that human reason is similarly legible. Note how closely Turing’s mental machinery resembles Watson and Crick’s central dogma, the neat sequential processing of a symbolic code, a biological algorithm that established a unilateral flow of information from DNA to RNA through transcription and then to proteins through translation.
Turing, however, was well aware that people’s mental lives would not be easily duplicated in machines. The human computer that had inspired the machine computer indulged in sensual pleasures impossible for the machine. In “Intelligent Machinery,” Turing also speculated on the body question. The intelligent machine he dreamed of “would still have no contact with food, sex, sport and many other things of interest to the human being.” Therefore it seemed best to explore what “can be done with a ‘brain’ which is more or less without a body, providing at most organs of sight, speech and hearing.”217 Turing thought that mathematics, cryptography, and human languages all seemed suitable to such research.