Commentary Magazine

The Dragons of Eden, by Carl Sagan


The Dragons of Eden: Speculations on the Evolution of Human Intelligence.
by Carl Sagan.
Random House. 263 pp. $8.95.

Among the ancients it was believed by some that the power of vision depended on invisible emanations issuing from our eyes and probing objects as blind people’s canes do. At the dawn of modern science in the 16th century, René Descartes scraped the opaque coating off the back surface of a bull’s eye and saw the upside-down image on the retina. To get the picture further in, to the inner sensorium, he credited the nerves. He believed that nerves were hollow tubes in which tiny filaments carried messages, in the case of mankind, to the seat of the human soul deep within the brain. Gusts of “animal spirits” blew down the nerves to the muscles to cause movement, said Descartes. Except for free will, the human body seemed to him a pneumatic machine. Pneumatic toys were in vogue in France in his time.

By the turn of the 20th century, science had passed well beyond Descartes. Nerves were known to be composed of tiny electrical cells, neurons, each supposedly capable of only two states, firing and not firing. Vision, for example, was thought by many scientists to be nothing but neurons firing in the visual part of a nervous system, often likened in the early 20th century to a miniaturized telephone switchboard. In fact, the entire human nervous system, with its billions of neurons, was supposedly nothing but a switchboard with a staggering potential for making connections (and presumably also wrong numbers). Carl Sagan (an astronomer who ventured far out of his field to write this popular psychology) supplements the telephone analogy with the by-now familiar idea that the cerebral hemispheres of the brain are like computers, a digital computer for one hemisphere and an analogue computer for the other. This analogy expresses the finding that speech and perhaps other logical or quasi-logical processes are localized in one brain hemisphere, while musical perceptions and perhaps other sorts of non- or pre-logical processes are localized in the other. The separation of functions into two hemispheres is not as sharp, or as well documented, as popular accounts would have it, but that need not occupy us here.

The quick scan of 2000 years of psycho-physiologizing should suggest two simple lessons about this subject. First, scientific analogies come from the available hardware, from canes to computers. In each age, people’s hunches about the physical basis of mind are limited by the physical knowledge of their time. Second, it would be rash to suppose that today’s analogies are here to stay. Speculation about the physical basis of mind has been going on for millennia without long resting with any particular theory. Indeed, Professor Sagan is a bit out of date, for scientists at the frontiers of psycho-physiology are thinking about chemical machines and holographic computers (neither digital nor analogue, but something quite new) as possible models for the still largely unexplained mysteries of the nervous system.

Physical speculation about the mind has never lagged far behind the evolution of hardware. At present, we still do not know the whole story about how nervous tissue achieves any psychological function except, in a few instances, the conversion of physical stimulation into sensory-nerve impulses. This has been no minor accomplishment, and has properly been celebrated with several Nobel Prizes and other signs of scientific preeminence. While science has learned many things about the nervous system itself—its architecture, its maturation, its chemistry, and its physics—I believe it is fair to say that we still do not know what kind of machine it is as it mediates a creature’s psychology. We are not even sure if its psychological functions are purely neuronal or whether other kinds of brain cells—the non-neural glia in particular—are involved. There has been plenty of progress—the brain is closer to a computer than to a pneumatic pump with valves—but there may still be a long way to go.



Like many non-specialist popularizers of psychology, Professor Sagan overestimates our physiological knowledge and underestimates our psychological knowledge. I’ll get back to this point later. First, I must acknowledge that Professor Sagan has taken the first hard step in learning psychology. The first step in studying psychology is to convince yourself that there is something to study, above and beyond common sense and common knowledge. Professor Sagan likes the theory of the “triune brain,” as formulated by a neurophysiologist named Paul MacLean in the early 1950’s. Not an active theory in the technical literature these days, it nevertheless appeals to popularizes. I used a bit of it myself in an introductory textbook I co-authored, though I did not give it nearly the weight it has here. The theory depicts the human brain as combining in uneasy equilibrium our reptilian ancestry, our pre-human mammalian ancestry, and our rational, competent selves. A reptile, a mammal, and a human reason within each skin—with these wild cards, Professor Sagan can play just about any hand he wants.

Tripartite psychologies are hardly new. Aristotle used three souls—the vegetative, the animal, and the rational; Sigmund Freud used the id, ego, and superego; W. H. Sheldon, the modern body-typer, uses three temperaments—viscerotonia, somatotonia, and cerebrotonia. I have not counted, but there must be at least a dozen major psychological systems based on tripartite divisions and very few based on any number other than three. Why is this? I’m not sure I know the answer, but I’d guess that it has little to do with the facts of matter.

Let us first consider the evidence for the triune theory. A tripartite division of the human brain has been used by certain anatomists: the cerebral cortex at the top, the lower brain stem at the bottom, and something called the limbic system in between. As anatomy, there is nothing compelling about this particular subdivision, rather than, say, the fivefold subdivision that I learned in my anatomy course in college. In fact, it is not even quite complete, for it omits the cerebellum, a large brain structure whose functions include at the least the coordination of movement. But granting the anatomy, there is still no reason for saying anything more than that the human brain is an evolutionary extension of earlier nervous systems, including not only mammalian and reptilian, but probably also those of fish, worms, and so on. But exactly the same thing could be said of the rest of our bodies. Our limbs harken back to the fins of fish, our bilateral symmetry to the bilateral symmetry of worms. Every species has the brain to go with the rest of its body, one that evolved over eons in specific environmental circumstances, and so it is with us. Only the loosest sort of pseudo-evolutionism would permit one to say that the lower brain of a human being is psychologically reptilian, and I am not even sure that it means anything in any event. It is one thing to have the brain of a rattlesnake in us and quite another if the reptile is a herbivorous dinosaur.



The first problem with this book’s tripartite psychology is, then, its inherent weakness. Another is that, as psychology as distinguished from anatomy, it is no match for Freud’s or Sheldon’s or even Aristotle’s. It appeals to brain anatomy while trying to say something about human behavior. They are not the same, although non-psychologists and beginning psychology students looking for a scientific psychology almost always gravitate to physiology and anatomy. Anatomy and physiology are tangible and technical, and biologists generally enjoy higher status in the academic pecking order than psychologists. It takes conscious effort and a certain amount of sheer study to resist the lure of physiologizing and to see that a creature’s psychology not only can, but must, be studied in its own right before scientific sense can be made of it. When he physiologizes his psychological hunches, Professor Sagan is investing his hunches with specious authority from a “harder” science.

The triune brain probably appeals to Professor Sagan because it seems to jibe with his psychological intuition. It captures in an evolutionary metaphor the idea that conflicting motives, conflicting values, and conflicting forms of knowledge are the human condition. Many of the other tripartite psychologies are also metaphors expressing more or less the same idea. Without further evidence, one metaphor is about as good as another, assuming we are after science, not poetry. The evidence must, moreover, be psychological. Except for a review of language in chimpanzees, The Dragons of Eden lacks psychological data. Besides anatomy, there is paleontology, geology, garnishes of astronomy, chemistry, and physics, but almost no psychology.



For psychology, Professor Sagan mostly relies on intuition. His intuitions are not unreasonable; he believes we possess both logical and alogical tendencies, that we are not as rational as we think we are, that we are conscious of only a fraction of the psychic forces that move us, that some of our psychology is adaptive for a world that no longer exists, that human relationships resemble to some extent relationships among other animals. But it is a pity that Professor Sagan has ignored the evidence that refines and transcends those sensible (though hardly original) hunches of his. Even more, it is a pity that Professor Sagan has ignored the rich, growing, often surprising and occasionally sobering discipline of psychology. But then, I have yet to stumble across a popularizer who knows much psychology, let alone bothers to popularize it. Fortunately for Professor Sagan, his book will not often be reviewed by a psychologist, judging from current practice in our leading newspapers and magazines. The editors who choose reviewers are generally even more innocent of psychology than the popularizers who write about it. They consequently share the impression that psychological “knowledge” consists mainly of intuition bolstered by casual observations and introspection. And they consequently will find nothing odd about a psychology book by an astronomer being reviewed by novelists, biochemists, and mathematicians. If the popular psychology conforms to popular political or moral lessons, so much the better. In contrast, psychologists who think their data could add something to common sense are often handled rather roughly when they surface above the technical literature, particularly when the message is unwelcome.

Professor Sagan can rest easy; his message will trouble very few (except the occasional psychologist who looks at the book). For example, Professor Sagan comments in the course of his discussion of the frontal lobes of the brain:

Cassandric components of our nature are necessary for survival. The doctrines for regulating the future that they produced are the origins of ethics, magic, science, and legal codes. The benefit of foreseeing catastrophe is the ability to take steps to avoid it, sacrificing short-term for long-term benefits. A society that is, as a result of such foresight, materially secure generates the leisure time necessary for social and technological innovation.

Indeed, but did we need to know the gross anatomy of the brain to say this? And what, in fact, are the bases of ethics, magic, science, and legal codes? How does an organism, human or otherwise, learn to sacrifice short-term for long-term benefits? What are the relevant abilities, and what are the limitations on the abilities? Would it surprise Professor Sagan to learn that there are data, real data, about the psychology (rather than the pseudo-physiology) of ethics, magic, and all the rest in this little passage? It would certainly surprise those of his readers who are relying on him for finding out about human psychology.

Psychology left the armchair for good at about the time of World War II, under the press of wartime needs for psychological testing, counseling, and the effective design of instruments and training procedures. There have been many elegant, perhaps occasionally important, discoveries in thirty years, and, of course, vastly more pedestrian work of no great moment. Psychologists are excited these days about research that measures the speed of thought (not nerve impulses, but thought itself), the information in mental (not retinal, but mental) images, the necessary and sufficient conditions for learning, the relationships among desire, gain, and action, the attributes of sensation, the percepts of animals, the relations between language and experience, the varieties and growth of moral reasoning, the dynamics of social encounters, the uses of reward and punishment in psychotherapy, and so on. A popular psychology should not only speak to non-specialists but should tell them things they do not already know, even at the risk of reshaping their views of themselves in relation to each other and to the physical and biological world around them.

Though he knows how profoundly science can change our picture of the world, Professor Sagan seems not to realize that the psychological shift in outlook promises to be deeper, broader, and at times more difficult to accept than the Copernican and Darwinian ones were. Once again, people are to confront a science that challenges their egocentricity, but this time it is their very sense of themselves and their personal destinies that are to be challenged, not such relatively secondary possessions as the planet on which, or the bodies in which, they live. The intensity of their resistance is already apparent in their alarmed reactions to B. F. Skinner’s conditioning procedures or A. R. Jensen’s assessments of individual differences in cognitive ability. None of this surfaces in, or even ripples the surface of, Professor Sagan’s book. He is asking his readers to change their minds about almost nothing, though doing so wih grace, humor, and style.

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