One afternoon in the fall of 1995, John McDonough tells us in his new book, "Experiencing Politics," he was sitting in his seat on the floor of the Massachusetts House of Representatives as the chamber prepared to vote on a huge tax break for Raytheon, the locally based defense contractor. McDonough thought the tax break was a terrible idea. He considered it a form of corporate blackmail. So he pushed the little red button on his desk and voted against it.

Then the majority whip came over to see him. "The Speaker wants a green from you on this," the whip said. That didn't make any sense to McDonough. The tax break was passing easily. His vote wasn't needed. The whip didn't answer his question. "He just does," she said.

If this were an ordinary political memoir, or if John McDonough were an ordinary politician, the rest of the story would be obvious at this point. McDonough would summon up his courage and stick with his beliefs. Perhaps he would be punished for his vote; perhaps not. But he would include the story in his book as a small profile in self- justification.

But "Experiencing Politics" is a very unusual memoir. And so the author tells us matter-of-factly just what he did on the Raytheon bill. He caved. A few months down the road, he had his own favorite bill coming up, and he wanted the Speaker's help. He didn't want to take any chances. He walked right back to his desk and switched his vote.

Then he did what he always does in difficult situations. He began pondering the philosophical implications of his choice. "Was my switch," he asks, "an example of naked, opportunistic self-interest or of a hard trade-off necessary to achieve a higher good? Anyone can characterize my action either way. The most honest answer is that both perspectives contain some degree of truth."

John McDonough is a compulsive framer of the most difficult questions, and a seeker of honest answers. He spent 13 years in the Massachusetts House unashamedly playing to win, and maneuvering to accumulate power. But all that time, he was quietly asking himself what it all meant. What is the ultimate relationship between politics and morality, or politics and the good life? Why do fundamental changes in public policy occur? Are there scientific rules and models that can predict the way political institutions will behave?

In the early 1990s, while serving as a senior legislator, committee chairman and member of the House leadership, he began studying for a doctorate at the University of Michigan. Ultimately he gave up his legislative seat and became a professor of public policy at Brandeis University, where he teaches today.

Scholar-politicians are not rare in American legislative bodies, but they are not unheard of. In most cases, they are academics driven to leave the classroom and try practical politics on their own. Sometimes they prove to be very good at it, like Woodrow Wilson, U.S. Senator Daniel Patrick Moynihan and Parris Glendening, the current governor of Maryland. More often, they find the transition from academic to political life hard to master, and remain on the backbench for most of their careers.

What is genuinely rare is a politician who gradually molts into a scholar. John McDonough did not start out in politics with a self- image as an intellectual. He graduated from Boston College in the early 1970s and spent a decade as a liberal activist, serving as a union organizer for the Amalgamated Clothing Workers and then as a lobbyist for a statewide tenants' rights group. His interests were entirely on the practical side: He had been fascinated all his life by James Michael Curley, the legendary Boston mayor of the early 20th century, for whom his Irish immigrant mother had worked as a maid and cook. His first campaign for the legislature, in 1984, wasn't built on musings about moral ambiguity. It was characterized, as McDonough makes clear, by untroubled notions of what the right answers were in state government. It was just a matter of organizing and strategizing to achieve them.

It is the transition from ideological confidence to constructive doubt, and the unfolding of an omnivorous intellectual curiosity, that make "Experiencing Politics" (published this month by the University of California Press) both a stimulating book and a delight to read. It is a series of inside stories about legislative life, told crisply and with a sense of humor, alternating with excursions into the work of Aristotle, Machiavelli, Hannah Arendt, Joseph Schumpeter, Saul Alinsky, Mao Tse-tung and a host of other thinkers--ancient, medieval and modern.

McDonough finds himself returning again and again to the question of why change occurs. Why do proposed ideas and reforms lie ignored for decades by the entire political system, then achieve popularity and enactment seemingly overnight, in a process that often appears more magical than rational? Why do other ideas capture the public mood and quickly gain an aura of inevitability, only to die a mysterious sudden death when they hit the legislative arena?

To understand these puzzles better, McDonough first turns not to political science but to the physical scientist Thomas Kuhn, author of "The Structure of Scientific Revolution." As many of us remember from introductory physics in college, Kuhn argues that the most important advances in science--the revolutions--don't occur incrementally, through the process of trial and error, but suddenly, at moments when an entire system of ideas expires of old age and inconsistency, and a new package of assumptions arises to take its place.

Over the years, thinkers in a whole range of fields, including government, have attempted to make use of Kuhn's ideas, some more successfully than others. At the shallow end of the pool, there was the fatuous "New Paradigm" movement of the early 1990s, in which congressional Republicans implied that their discovery of the free market was somehow comparable to the pathbreaking insights of Copernicus or Einstein.

McDonough is careful not to fall into the "New Paradigm" trap. But he does find merit in the notion that public policy in any major field proceeds through a Kuhn-like cycle: quick, sharp bursts of energy and societal change (the New Deal, for example), followed by long decades of inertia and apparent equilibrium, and then by another burst of change (the deregulation movement of the 1980s). Not to see this, in his opinion, is to misunderstand what is going on at a given moment-- and quite frequently, to waste effort on schemes and projects whose timing dooms them to failure.

McDonough is not the only writer who has noticed these similarities, and he gives full credit to scholars who have observed them before, notably the political scientists Frank Baumgartner and Bryan Jones. What is original is McDonough's application of the ideas to an effort in which he was intimately involved: the battle to control medical costs in Massachusetts. He goes back to the 1950s to follow the triumph of an old idea (hospital rate regulation), the institutional inertia that came to surround it, the emergence of a new approach (managed care) and the strikingly sudden triumph of the new approach and discrediting of the old one. No theory is perfect, McDonough says, but a little knowledge of Kuhn helps in situations like this--to "shed light on those brief, rare moments when big change is not only possible but almost inevitable."

Even with Kuhn's assistance, however, there remains the question of how one can know that the time for change is ripe--how the practicing politician can tell that he is advancing his idea at a moment of genuine possibility, and not wasting his time pursuing revolution when the inertia of the system is too strong for him to succeed.

One answer, of course, is that this requires acute political instincts, and not everyone is born with those. But McDonough, looking for a more rigorous solution, turns to the work of John Kingdon, a political scientist widely admired within his field but little read by politicians and public officials.

Nearly two decades ago, Kingdon proposed that political reform occurs during brief "windows of opportunity." At those moments, three forces come together. One is the consensus among opinion makers that a legitimate societal problem exists. Another is the existence of a practical and comprehensive public-policy solution. And the third is the mood of the electorate. Kingdon says the stars come into alignment on any subject rarely and briefly, and if the opportunity is missed, the chance may not come again for decades.

Arguments can be made for or against Kingdon's thesis, but McDonough puts it to constructive work, using it to explain the failure of the Clinton health care plan in 1993 and 1994, and the enactment of a law expanding children's medical coverage in Massachusetts in 1996. In McDonough's view, a window opened for the passage of a national health insurance bill in 1992 and early 1993, but by taking nearly a year to draft a proposal, the Clinton administration squandered it. Four years later, there was another brief opening at the state level in Massachusetts, and in this case, the proponents of health care reform managed to squeeze legislation through it.

No one will agree with all the theories McDonough tries out in "Experiencing Politics"--indeed, as he admits, some of the ideas contradict each other. But it's hard not to admire McDonough's willingness to take up the most important theoretical issues in public life, and his patience and fairness in seeking to resolve them

Political scientists, in general, are people who ask all the crucial questions but have limited opportunity to find the answers. Politicians, too often, are people who know all the answers but have trouble remembering what the questions are. A practical politician comfortable in the realm of theory and ideas is an accident worth celebrating.