RICHARD A. POSNER is one of the more thoughtful judges on the federal bench today. He may also be one of the more important. The two adjectives are related. Judge Posner is always mentioned in any serious speculation about future Reagan appointees to the Supreme Court, and his ideas are often described in ways that set off alarm bells.

His principal idea is that judges ought to use economic theory in deciding cases. A corollary is that lawyers and law school professors, as well as judges, ought to incorporate some economics and political science into their legal theory.

Discussion of ideas like these is usually confined to universities and their law schools. But Posner's thesis has been reaching a wider audience. That is probably a good thing, although I suspect he has sometimes been dismayed by the way his thoughts have been capsulized. This rather heavy book makes it easy to see how his intense analysis could be converted into an intellectual justification for writing right-wing political and economic ideas into the Constitution.

Posner would apply these ideas, developed in large part when he was a professor at the University of Chicago Law School, to areas of the law as different as antitrust and the rights of convicted criminals. The latter application has led him to argue for curtailing those rights in ways perceived to be anti-civil libertarian.

His ideas do sound a bit peculiar. Lawyers have been taught that the law grows from logic and experience, not from the social sciences with their somewhat less formalized system of analysis. And informed laymen hear the ghosts of a generation of Supreme Court justices who tried, fortunately with only meager and short-lived success, to make Adam Smith a co-author of the Constitution. Any idea is bound to meet with skepticism if it allows the interpretation that constitutional questions should be answered in terms of costs and benefits, balance sheets and national income accounting.

But that's not exactly what Judge Posner has in mind. What he outlines in this book is his view that judges should use the essence of economics -- he describes it as "the theory of rational choice in a world of limited goods" -- when they are making some of the choices our system of government puts in their hands.

There are some areas of the law, particularly its administration, where the idea has a great deal of merit. Judge Posner makes a powerful case that a constantly increasing workload has brought the federal court system to a critical point. Using the tools of economic analysis, he demonstrates the scope of the problem and evaluates the ideas for reform that have been discussed in recent years.

HE APPLIES a cost/benefit ratio to subjects as different as the fees charged to file a lawsuit and the jurisdiction of the federal courts. But most of his costs and benefits do not have monetary values. They include such things as the quality of the federal courts and the interests of state and national governments in different kinds of cases.

These ratios lead him to support most proposals for reducing the workload of the federal courts -- raising the amount of money that must be in dispute before certain cases can be filed there, sending some admiralty cases to the state courts, and so on. Included in his list, I think unnecessarily and unfortunately, is curtailing even further what remains of the lower federal courts' authority over state criminal cases.

He does not, however, favor an additional court between the Supreme Court and the Courts of Appeals. Instead, he proposes a dozen or so ways in which the existing courts -- particularly the Courts of Appeals like the one he sits on in Chicago -- could help solve the present problems.

All this is useful and interesting but doesn't tell us much about how Judge Posner would act if lightning struck and he were elevated to the Supreme Court. He gives us many clues, though, in several chapters on the art and craft of judging. They suggest that he is, to use terms he intensely dislikes, a highly-principled, old-fashioned conservative more in the mold of the second Justice Harlan than that of Justice Brandeis -- who first brought real economics, not just laissez- faire rhetoric, to the Court.

Many of the things he says about judging will not go down well with either his peers on the Courts of Appeals or his superiors on the Supreme Court. He is highly critical of the way they write opinions (too verbose and too imprecise), of their public personal disagreements, of their reliance on increasing bevies of law clerks, of the disintegration of the courts as collegial bodies, and much more.

Some of his harshest words, however, are reserved for the law schools. They are so enchanted with the case method of teaching from appellate opinions that neither professors nor students can break out of that narrow focus on what the law is to think about what it ought to be. As a result, he says, there is little real scholarship or intellectual activity in the law schools these days.

The most impressive aspect of this book is the bluntness with which Judge Posner writes. He attacks with vigor one myth after another, including not only the rationales judges use to justify their decisions but even the essentially meaningless words (like self- restraint and neutral principles) that judges and their critics are so fond of throwing around. It is refreshing to have an able judge speak so candidly about the process through which judicial decisions are made.