Artificial Intelligentsia

THE CAMBRIDGE QUINTET: A Work of Scientific Speculation
By JOHN L. CASTI
Addison-Wesley
181 pages
$23 hardcover
Steve Allen once created a short-lived television series called Meeting of Minds. A deadpan Allen would introduce four or five guests, in period dress, drawn mostly though not exclusively from the ranks of the intellectual Immortals. Marx, Spinoza, Leonardo and Marie Antoinette were a typical quartet; Poe, Jefferson, Aristotle and Virginia Woolf were another. In all - well, not all - seriousness, the group would carry on for an hour the Great Conversation that is Culture. I was generally on the floor after 10 minutes.

John Casti is not, like Steve Allen, a comic genius, just a veteran science writer with a streak of whimsy. But The Cambridge Quintet is an inspired conception. One rainy night in 1949, in his (and Charles Darwin's) former rooms at Cambridge University, C.P. Snow, novelist and science adviser to the British government and soon to be famous for his pamphlet The Two Cultures, has arranged a meeting of minds. The government wants to know whether there's anything to the talk just then beginning to be heard about the possibility of "thinking machines." Accordingly, Snow has invited J.B.S. Haldane, Erwin Schrodinger, Alan Turing and Ludwig Wittgenstein to dinner. Over seven courses (and chapters) accompanied by sherry, Montrachet, Burgundy and cognac, the conversation unfolds.

The four guests are, of course, bona fide Immortals. Haldane was an early and pioneering population geneticist, a prolific popularizer and a leading (though eventually disillusioned) British Communist. Schrodinger was one of the founders of quantum mechanics, a dabbler in Eastern metaphysics and author of the now-classic little book What Is Life? Wittgenstein was not only one of the most influential thinkers of the 20th century but also one of its most influential personalities: ascetic, mystical, a kind of philosopher-saint. Turing, as much as anyone, launched the computer age with a series of revolutionary mathematical papers. (He also, as much as anyone, defeated the Nazis by breaking their previously impenetrable military-communications code, "Enigma.")

What started the rumors that so intrigued the British government was the invention of the "Turing machine." This was not a concrete object, with bells, whistles and wires, but rather the idea of an object with a certain structure and certain abilities. A Turing machine is something that can take in a symbol and perform a specified sequence of operations (called a "program") on it, thereby transforming it into another symbol, which it prints out. Logics, grammars and mathematical functions are also ways of processing symbols. Turing showed that symbol processing has a general structure (called "computation"), which can be embodied physically, in a machine.

But, Snow asks, is computation the same as thought? Our dinner guests are off and running. In one of several attempts at explaining the implications of his mathematical findings (like Godel's "uncertainty" principle and Schrodinger's "indeterminacy" principle, Turing's "computability" concept is so abstract and general that it has myriad implications and is susceptible of many different formulations), Turing remarks, "Anything at all that can be thought of as following a set of rules . . . can be calculated in a similar step-by-step fashion by this kind of machine."

This pushes the argument back a stage. Does the brain "follow rules"? Can its operations be formalized or made explicit? What is this thing called thought? Apparently, it's a conversation among neurons. The brain and nervous system are made up of roughly 10 billion neurons, or nodes, connected by wirelike axons and dendrites, which conduct electrical impulses to and from. "Something like a giant telephone-switching network," as Haldane puts it. The neurons have a "threshold of excitation": If the incoming electrical impulses are strong enough, they fire off impulses in turn, which are carried to other neurons. The "on-off" pattern of all the neurons (or some subset of them) at any one time is a mental state - in effect, a thought.

All this is strikingly analogous - in some ways, at least - to how a computer works. A neuron is like a computer "bit," or storage location; "on" and "off" correspond to the zeroes and ones of the binary system; stimuli to the nervous system are the equivalent of inputs to the machine; the firing of neurons and their rearrangement into new patterns resembles the executing of instructions from a program and the consequent rearrangement of the stored data into new configurations.

That's as much of the scientific logic as this reader could follow. Fortunately, there's also plenty of nontechnical talk. One of the more illuminating strands of the discussion, spread over several chapters, counterposes two famous thought experiments: the "Turing test" and John Searle's "Chinese Room" argument. (Searle's argument, actually published in 1980, appears in The Cambridge Quintet, renamed and attributed to Wittgenstein, through fictional license.) Turing proposed a simple and ingenious test for machine intelligence. Put a person in one room, a computer in another, both connected by teletype to a person in a third room. The person in the third room types in questions, to which the other person and the computer type out replies. If the questioner cannot tell consistently which replies come from the other person and which from the computer, then the computer is intelligent. If it talks like a duck . . .