John Kenneth Galbraith recently hailed it as one of the 100 most important books in the history of the world. It is undoubtedly the best-selling textbook of all time. And if, in recent years, Economics -- the 11th edition was published last month -- had fallen out of its position as the best-selling economics textbook on campus, Paul Samuelson is not concerned.

The Noble Prize winner, architect and grand old man of postwar, post-Keynesian economic theory, is a small man with merry eyes, a yellow bow tie and a piping confident voice. In his office at Massachusetts Institute of Technology, copies of all the editions of Economics -- and the dozen-odd foreign translations -- cluster in his bookshelves like pictures of devoted grandchildren.

Three to four million copies of the book have been sold since it was first published in 1948. Samuelson, one of the acknowledged masters of mathematical economics, grows uncharacteristically forgetful when asked how much money the book has brought him -- though he hastens to point out that "much of that was shared with my government" during a time when the maximum tax rate was 90 percent.

Economics embodies the conventional wisdom -- ideas that were once controversial and have survived to become banal. It takes its inspiration from John Maynard Keynes, the British economist who first theorized that a country could spend its way out of a depression by running budget deficits and might thereafter avoid recession altogether by spreading deficits over the downward swings of the normal business cycle.

The volume itself, over the years, has taken on the high style and self-assurance of its author. With its three colors of text type, four-color charts and agate appendices with names like "Geometrical Analysis of Consumer Equilibrium," it leads the apprentice economist on a march through the length and breadth of his new discipline -- beginning with a definition of the science and progressing to concepts like marginal physical product and Pareto optimality.

When the trip is over, the iron laws of scarcity, starvaton and depletion have by imperceptible degrees taken on a benign and sunny aspect, a human face taht looks much like that of the book's creator. For example, the 10th edition assures us on page 373 that "a mixed economy can avoid the worst excesses of boom and slump . . . The broad cleavage between microeconomics and macroeconomics is closed by active public use of fiscal and monetary policy (save for the unsolved dilemma of stagflation in the mixed economy!)"

Save for the unsolved dilemma . . .

Samuelson cheerfully volunteers that since the 10th edition saw print in 1976, the problem of stageflation has revealed itself to be more difficult than he had thought. In the new edition, "stagflation has to be not just a problem for the third to the last chapter -- it has to be right in the beginning. There's nothing for it but to sit down at the typewriter and start anew."

Some critics have suggested that the Western world might have to do that with the whole of post-Keynesian economics -- that the optimistic conventional wisdom became obsolete when the boom of the '60s died and should be replaced with some new doctrine: the austere teachings of "small is beautiful," the bold simplicity of the Kemp-Roth tax cut, the complexity of some new form of socialism.

Samuelson admits that possibility, but he is not concerned that it will happen soon. "The worse the economy behaves, the less complacent we economists feel, the bigger the enrollment in the economics classes, the greater the speaking invitations, the higher the fees," he says comfortably. "Funeral by funeral, science advances. But it doesn't happen out of the blue. There are successes of the new paradigm before it replaces the old.I haven't seen that."

Constant revision of a massive text is quite a bit to take on for a man who could relax and enjoy his distinction, especially for a busy scholar who must also teach three classes a year, write 17 columns for Newsweek, issue two half-hour cassettes of commentary a month and provide occassional articles to the Financial Times of London and Nihon Keisai Shimbun of Tokyo. What's in it for him?

"You tap a different level of fame," he said with a smile. "Whenever I've taken one of my numerous children into Mass General Hospital after a football accident, there's always an intern who'll say, 'I used your book at Yale' -- so you get a leg up on the triage process. Besides, who was it who said, 'I care not who writes a nation's laws as long as I can write its songs'?"