During the Renaissance, when philosophers had a surer sense of the eternal scheme of things, a widely-cited maxim offered the wisdom that man proposes but God disposes. These days, in the more temporal world of Washington, it's the president who offers and the Congress that decides. A case can even be made that now it's the Congress that both proposes and disposes, dispensing with the president entirely.

That's an overstatement, of course. But if there's any doubt the continuing difficulties Jimmy Carter is having with Congress, consider the example of the president and one of the most powerful congressional chairman. No sooner had the president again spelled out his proposals ofr revising the tax system than Ways and Means Chairman Al Ullman (D-Ore.) was giving his version of what was going to happen. "We're going to have a different mix, he immediately proclaimed, indicating that much of the Carter tax proposals would be scrapped.

That was last Monday. Two days later Ullman was proposing again. He was offering a substitute plan to Carter's welfare revision ideas. His version was quite different: it would cost some $12 billion less, and offer less relief to the states. These aren't the only times that Carter and the chairman have differed on major issues. Their approaches on how best to guarantee the financial integrity of the Social Security system were at opposite poles.

In a different political era, these clashes could be seen as a classic Washington power play, the inevitable collision of opposite ofrces on either end of Pennsyivania Avenue. The strong chairman vs. the strong president, Barah and FDR Lodge and Wilson.

But the Ullman-Carter situation is more subtle and significant. It's not ideology that's at issue, nor even so much philosophy. It's their political approaches - how to achieve the art of the pratical - that are most striking.

At face, they much in common. Ullman, a key Democrat, now deals for the first time as chairman of a critical committee with a president of his own party. Like Carter, he's not identified as a strong liberal or conservative. During his 21 years in Washington, he's earned the reputation as a mediating influenece, a middle-of-the-roader, no public brawler. His easygoing, calm manner belies the impression of the cantangerous congressional baron of old.

In fact, Ullman knows well that the day of overpowering figures on the Hill - LBJ, Rayburn, Dirksen, Kerr - are past. His immediate predecessor as committee chariman, Wilbur Mills, was one fot eh last of that breed.

"It's not possible, as it used to be, for a chairman to carry enough votes in his hip pocket to go around and make a lot of promises," he says. "I can't do that."

His committee like the Congress itself, has changed. The membership has been expanded from 25 to 37. Among them, they now fill six separate subcommittees. The professional staffs are larger, the members more independent.

And Ullman takes pams not give the impression that he's opposed to Carter's general long-range goals - a stablized economy, and energy program, tax and welfare revision. It's over how those goals are achieved that they differ, and their political differences are instructive.

A meeting had just broken up in the ornate second-floor room overlooking the Capitol Plaza. Ullman clear away his papers and began talking about his relationships - and differences - with Jimmy Carter.

"I think Carter's a decent, dedicated man who wants to do the right things for the country," he said. "I think he's had a hard time - not just because of himself, but because of some of the people around him - in adjusting to the format and the procedures that involve the whole processes of government. It isn't what a president recommends. It's what he gets through the Congress that counts.

"In order to understand that, you have to understand the nature of the Congress and the nature of public opinion that the Congress represents! You have to understand the whole background of concepts that go into the making of opinions of Congress. I think that my role is to try to project that, to take the things the president recommends and make them over into a format that fits the capacities of the Congress. And timing is terribly important.

"That's one thing I think the President hasn't understood. Timing is critical. It's absolutely essential. In taxes, for example, what you couldn't do last year you can do this year. It's terribly important to build on some knowledge and expertise and experience, and I think the president kind of failed to take some of that into consideration."

Ullman says he finds Carter an excellent listener, eager to get other views, and readily available. He is not an isolated president.

"But my problem isn't talking to the president," he adds. "It's that the decision-making process of the White House seems stuck. My conversations aren't at the level where the decisions are finally made. By the time those decisions finally come out there have been all of these other characters in the act that have muted and nullified the changed them."

Welfare was case in point. For years Ullman had been listening to the arguments advanced by some sociologists, economists and social planners about establishing a guaranteed annual income as a solution. Lyndon Johnson didn't buy the idea. Richard Nixon did, but it was never implemented. Ullman concluded that such an approach "would ultimately destroy the foundations of this country if we just opened up the treasury to individuals by guaranteeing them incomes." Then the Carter welfare proposals came out. Ullman was disenchanted.

"Now glory be," he says," the president came up with the same solution. Part of his program is another guaranteed income. I told him I couldn't buy it before he introduced it. I absolutely abhor it. Therefore, I have a responsibility to try and develop an alternative."

He's done so now with both welfare and taxes.

One of the more fascinating aspects of this relationship - and, by extension, to the entire Carter-Congress role - is the common note of political frustration sounded.

For a year Jimmy Carter has been attempting to summon the nation to the realities of an energy crisis, to dealing with such intractable old problems as taxes, welfare and revamping the federal establishment. Ullman, too, talks about the difficulties of trying to lead in these confusing times.

"That's one of the problems of democracy," he says. "If we had a visible crisis - war, depression, disaster, flood, whatever - we'd have all of the people behind us. But when you have an invisible crisis of energy, inflation, even welfare, it's very difficult to face up to it because it's confusing and little understood. That's our problem now - how we can make hard decisions when the crisis, although real, isn't felt."

His personal solution is to continue standing firm and offering his own proposals when he feels it necessary. So, it's virtually certain, will Jimmy Carter. But then this is a story without an end.