Taking a bit of journalistic license, one can divide the 20th century neatly into scientific epochs. The first quarter was the heroic age of theoretical physics, of relativity and quantum theory: the Age of Einstein. The second quarter saw the exploration of the atomic nucleus, with fateful consequences: the Age of Rutherford and Bohr. In the third, life yielded its immemorial secrets to molecular biology: the Age of Watson and Crick. In the fourth, the study of mind -- cognitive science -- was born and has grown to maturity, or at least into a strapping adolescence. It dates, more or less, from the discovery of universal grammar, so call it the Age of Chomsky.

In the era B.C. (Before Chomsky), the study of mind was largely a matter of trained pigeons and Oedipus complexes, while linguists roamed around Babel, describing, comparing, classifying, and compiling. Chomsky brought order into Babel. He noticed that, at a deep level, all languages had complex but remarkably similar logical features: procedures for combining linguistic elements, transforming one element into another, embedding one within another, etc. Normal language users follow these procedures, unerringly, but without any explicit knowledge of them. Children use them, not only without being taught them but before they’ve even heard other people use them. It gradually dawned on Chomsky -- and since then most of the scientific world has come to agree -- that language is not learned.

But what else could it be? We’re not born talking. True, but neither are we born running or chewing or menstruating, and we eventually do all these things, without ever learning to. Language, Chomsky proposed, is innate. Not, of course, every last grammatical rule and stylistic nuance -- those have to be learned. It’s the logical structure, or “universal grammar,” that we are born with, a mental organ that just grows, like the heart and lungs.

This idea is -- to put it mildly -- counterintuitive and has long stood in need of a good popular exposition. Steven Pinker’s The Language Instinct: How the Mind Creates Language (1994) more than filled the bill. It is a classic of popular science writing: comprehensive, clear, lively, funny, and provocative. He has now followed it up with an even more ambitious volume, How the Mind Works, which is, perhaps inevitably, not quite as good. But almost.

The mind, Pinker writes, is “a system of organs of computation designed by natural selection to solve the problems faced by our evolutionary ancestors in their foraging way of life.” The “computational” theory of mind explains how information (i.e., symbols arranged in patterns) causes physical events. This very general and powerful idea is the basis of present-day cognitive science, and Pinker’s early chapters may be the best chance non-specialist readers will ever have to understand it. The second half of Pinker’s definition invokes Darwinian natural selection, and here too the exposition is first-rate, though there are many more competitors.

As Pinker tells it, specific mental skills or modules, like depth perception and pattern recognition, arose in response to one or another survival difficulty on the savannah, where our species started out roughly two million years ago. Hunting, gathering, climate, predators, sexual rivalry, etc. generated the challenges. Beginning with what we currently know about the brain’s structure, circuitry, and performance, and taking into account the fossil record, anthropological data, and animal physiology, we can reconstruct for each response, or module, a possible developmental history. This process is known as reverse engineering -- “in forward-engineering, one designs a machine to do something; in reverse-engineering, one figures out what a machine was designed to do” -- and is the basis of the new sciences of evolutionary psychology and cognitive neurophysiology.

But how do we know that the brain is a machine? And what are the moral implications of saying so? Answers to these and dozens of other $64,000 questions, from “do we think in words or images?” to “how much is nature and how much is nurture?” to “what is sex appeal?” to “why is music pleasing?” -- along with more laughs than one would have thought possible in so profoundly illuminating a book -- await readers of How the Mind Works.

Readers of Terrence Deacon’s The Symbolic Species: the Co-Evolution of Language and the Brain will have to work harder than Pinker’s. Like Pinker, Deacon ranges across several disciplines; but while Pinker is primarily a psychologist, Deacon’s home base is neurobiology, which leaves fewer openings for quoting Woody Allen jokes, Steve Martin movies, and rock lyrics. Occasionally one runs up against sentences like this: “Displacement of midbrain axonal connections by cortical efferents likely includes prefrontal projections to midbrain and brainstem nuclei that also receive limbic and diencephalic inputs and contribute to stereotypic vocal displays in other species.” There are no sentences like that in Pinker.

Still, The Symbolic Species is, if possible, even more rewarding than How the Mind Works. Actually, though the two books were written simultaneously, the former is in a sense a reply to the latter. Deacon accepts the central Chomskyan insight: the grammatical knowledge possessed by all normal children is too complex to have been acquired by inductive learning. The apparently inescapable Chomskyan conclusion is that this knowledge is a priori, hard-wired in children’s brains: a “language instinct.” Deacon demurs. The basis of language acquisition is “neither in the brain of the child nor in the brains of parents and teachers, but outside brains, in language itself.” Language is “its own prime mover ... the author of a co-evolved complex of adaptations arrayed around a single core semiotic innovation.”

This crucial innovation is “symbolic reference.” To condense an already very dense hundred pages or so: symbolic reference is a relation between sign and object that is independent of the characteristics of either (like, say, word and object; and unlike, say, a statue and its subject or a thermometer and the air temperature).Language is one kind of symbolic reference.

Only humans employ symbols. Why? For Deacon, the answer to that question is the key to the deepest mysteries of language and mind. As he pursues the answer through two million years of fossil remains and down every neural pathway in the brain, readers who stay the course will find the suspense building palpably. Unfortunately, not all readers will persevere -- the book’s technical detail and conceptual rigor are daunting. At the end, though, comes the vertiginous excitement of glimpsing, in the distance, the solution to more than one of philosophy’s perennial problems.

And not a moment too soon. This has not been an altogether inspiriting century (or millennium) for fans of Homo sapiens. In that perspective, Pinker and Deacon are bearers of reassuring news -- not about human nature, but about at least one human activity. The vast cooperative intellectual enterprise we come to know by spending a month with these two books is simply thrilling in its intricacy, subtlety, and scope. Whether or not it definitively explains our species, it at least goes a little way toward toward redeeming us.

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