WHEN GARRY WILLS, in his Nixon Agonistes , invited us to consider the proposition that Nixon was a liberal, we appreciated the jest, but we didn't take it too seriously. It might be true that there was a streak of 19th-century liberalism under the southern California tan. But for all practical purposes we went on calling Nixon a conservative. And so did Nixon.

Now, in calling himself a conservative, Wills seems to be playing with tht same paradox. After all, didn't Wills mercilessly dissect Richard Nixon, denounce the Vietnam war, write for The New York Review of Books ?

One of Wills' purposes in this book, admittedly, is to burn off some of the conceptual confusion of political discourse in the U.S. - to restore precision to what he calls "a topsy-turvy world of terms."

Those who are called liberals, he reminds us, have little or no inheritance from historical liberalism. Similarly, it is mindless to give the name of conservatism to "a hodgepodge of anarchic and repressive resistance to our liberal establishment."

When Wills calls himself a conservative, he draws a careful distinction. Most of those who are known in common parlance as conservatives, he calls "ring wing." And Wills calls himself a conservative not merely as an attention-catchiang device or to persuade us that the term is wrongly used.

He comes out of a conservative stable for a start. Wills began his career as the drama critic for William F. Buckley's National Review . But like so many others of our generation, he was forced to rethink his political philosophy by the civil rights movement, especially by the questions of civil disobedience which it raised, and of course above all by the Vietnam war.

"Yet I could not think of this [rethinking] as a departure from conservatism," he writes, "but rather as a late arrival at it. The belief that one's own nation has to defend justice in every other nation is not conservative or liberal so much as lunatic."

At one level Wills' book is the account of an intellectual journey, modeled on the confessions of his favorite St. Augustine. But for Wills the more important purpose of the book is an exploration of territory where not all readers will be so keen to follow him: into that obscure and dangerous realm where political philosophy, moral philosophy and religion meet.

Wills argues for what he calls an "accommodationist" theory of politics. The state is not justice. It should not be founded on deep commitments to principle. It should try to make it possible for people with strong, potentially incompatible beliefs to live together peaceably. This is, as Wills admits, hard doctrine for a nation dedicated to a proposition, and holding truths to be self-evident.

Wills espouses that doctrine for reasons connected with his own Christian belief: or so it appears, though he does not say so in so many words. "For those whose grasp of the transcendence of God makes them deny the possibility of replicating that purity in the state, politics can only be accommodationist."

This is the book of a wise and civilized man, who has things to say and knows how to say them. But it is not altogether a successful book. Perhaps he has too much to say. It moves too much at the writer's pace, not at the reader's, from autobiography to political philosophy to what the learned Mr. Wills might call ta eschata . I closed it with slight intellectual indigestion, and a feeling that he had profligately tossed the ingredients for three very good books into one casserole, where their flavors tend to kill one other.

Jack Anderson's Confessions , readers of "Washington Merry-Go-Round" might expect, would be of a distinctly less Augustinian kind, even though it is known that he is in his private life a most abstemious Mormon. Still, the column he inherited from Drew Pearson has never shrunk from telling its readers what they as citizens need to know about what Anderson calls "the gamy pratfalls" of the mighty.

With an eagerness of which I ought to be ashamed, I looked forward to a whole vaudeville of Washington's latest pratfalls. But no such luck. These aren't Jack Anderson's confessions at all. In spite of its title, the book is essentially a memoir of Drew Pearson during the period from 1947 to 1969 when Anderson worked for him, sauced with some of Anderson's reflections on the ethics and practice of investigative journalism as he learned it from Pearson.

The substance of the book consists of highly readable narratives of many long-forgotten scandals - several of which deserve to be better remembered, such as the strange beliefs and sad end of James Forrestal, and the press's almost unanimous willingness to conceal from the American public the fact that the power of the Pentagon was in the hands of an acute paranoic psychotic.

Anderson muses in a not particularly profound vein on the ethics of the business which he has practiced with unrelenting enthusiasm for the past 30 years.

He is, he says, troubled by the cost of the investigative reporter's triumphs, by the two edges of the sword he wields.

"Every success of the investigative reporter," he believes, "means ruin for some human being who is typically weak rather than evil. Most of the time I am militantly convinced that the trade-off is necessary to maintain a free society. But there are seasons when it seems a close call."

Anderson writes with what I take to be implicit disapproval that "where his causes and the resulting conflicts were concerned," Pearson was "girdled with an implacability that could not be pierced by sentiment nor softened by tragedy." He also felt that Pearson was "skewed by his sense of protectiveness towards the liberal-left": not a problem for Anderson, who got on like a house on fire with Joe McCarthy, and indeed gave him access to Pearson's files without asking Pearson, on his own account of the matter.

There is no question that Drew Pearson could be implacable where his causes were concerned, and no question, either, that he was quite good at persuading himself that his own enemies were the Lord's.

But my impression, I am afraid, is just the opposite of Anderson's. I think the reason why Pearson was, like it or not, a master where Anderson is a journeyman, and why Pearson had vast moral and political influence, while Anderson has less than that, is because - relentless old scold and cunning operator that he was - Drew Pearson did what he did out of political beliefs. In contrast, Anderson appears to have only the simplest political ideas and to think of himself in a sort of business world.

That's no sin, no doubt. But a journalist who fights for causes is perhaps in the end less likely to end up destroying weak and harmless creatures unnecessarily than one who is concerned with maximizing his kill-ratios. CAPTION: Picture 1, Gary Wills, Copyright (c) 1979 Jill Krementz; Picture 2, Jack Anderson, AP