Every person carries within his head a mental model of the world – a subjective representation of external reality. This model consists of tens upon tens of thousands of images. These may be as simple as a mental picture of clouds scudding across the sky. Or they may be abstract inferences about the way things are organized in society. We may think of this mental model as a fantastic internal warehouse, an image emporium in which we store our inner portraits of Twiggy, Charles De Gaulle or Cassius Clay, along with such sweeping propositions as "Man is basically good" or "God is dead."

Any person's mental model will contain some images that approximate reality closely, along with others that are distorted or inaccurate. But for the person to function, even to survive, the model must bear some overall resemblance to reality. As V. Gordon Childe has written in Society and Knowledge, "Every reproduction of the external world, constructed and used as a guide to action by an historical society, must in some degree correspond to that reality. Otherwise the society could not have maintained itself; its members, if acting in accordance with totally untrue propositions, would not have succeeded in making even the simplest tools and in securing therewith food and shelter from the external world."

No man's model of reality is a purely personal product. While some of his images are based on firsthand observation, an increasing proportion of them today are based on messages beamed to us by the mass media and the people around us. Thus the degree of accuracy in his model to some extent reflects the general level of knowledge in society. And as experience and scientific research pump more refined and accurate knowledge into society, new concepts, new ways of thinking, supersede, contradict, and render obsolete older ideas and world views.

If society itself were standing still, there might be little pressure on the individual to update his own supply of images, to bring them in line with the latest knowledge available in the society. So long as the society in which he is embedded is stable or slowly changing, the images on which he bases his behavior can also change slowly. But to function in a fastchanging society, to cope with swift and complex change, the individual must turn over his own stock of images at a rate that in some way correlates with the pace of change. His model must be updated. To the degree that it lags, his responses to change become inappropriate; he becomes increasingly thwarted, ineffective. Thus there is intense pressure on the individual to keep up with the generalized pace.

Today change is so swift and relentless in the techno-societies that yesterday's truths suddenly become today's fictions, and the most highly skilled and intelligent members of society admit difficulty in keeping up with the deluge of new knowledge – even in extremely narrow fields.

"You can't possibly keep in touch with all you want to," complains Dr. Rudolph Stohler, a zoologist at the University of California at Berkeley. "I spend 25 percent to 50 percent of my working time trying to keep up with what's going on," says Dr. I. E. Wallen, chief of oceanography at the Smithsonian Institution in Washington. Dr. Emilio Segre, a Nobel prizewinner in physics, declares: "On K-mesons alone, to wade through all the papers is an impossibility." And another oceanographer, Dr. Arthur Stump, admits: "I don't really know the answer unless we declare a moratorium on publications for ten years."

New knowledge either extends or outmodes the old. In either case it compels those for whom it is relevant to reorganize their store of images. It forces them to relearn today what they thought they knew yesterday. Thus Lord James, vice-chancellor of the University of York, says, "I took my first degree in chemistry at Oxford in 1931." Looking at the questions asked in chemistry exams at Oxford today, he continues, "I realize that not only can I not do them, but that I never could have done them, since at least two-thirds of the questions involve knowledge that simply did not exist when I graduated." And Dr. Robert Hilliard, the top educational broadcasting specialist for the Federal Communications Commission, presses the point further: "At the rate at which knowledge is growing, by the time the child born today graduates from college, the amount of knowledge in the world will be four times as great. By the time that same child is fifty years old, it will be thirty-two times as great, and 97 percent of everything known in the world will have been learned since the time he was born."

Granting that definitions of "knowledge" are vague and that such statistics are necessarily hazardous, there still can be no question that the rising tide of new knowledge forces us into ever-narrower specialization and drives us to revise our inner images of reality at ever-faster rates. Nor does this refer merely to abstruse scientific information about physical particles or genetic structure. It applies with equal force to various categories of knowledge that closely affect the everyday life of millions.