Everyone knows where Ronald Reagan stands on the important issues of our time. Few, including those who have worked for Reagan throughout his public career, understand the process by which he reaches his decisions.

This decision-making process is the central mystery of the Reagan presidency. Though the president passes up few public opportunities to reiterate his fundamental beliefs, he almost never explains how these views are translated into complex policy decisions. Even in the several extensive interviews he has given, Reagan keeps his thought processes to himself.

In fact, no modern president has explained himself as little as Reagan. Other presidents as different in personality as Jimmy Carter and Gerald R. Ford enjoyed expounding on their decision-making processes. Lyndon B. Johnson in particular would hold friends, reporters and advisers captive for hours as he regaled them with the thoughts of LBJ.

But public introspection and analysis are not Reagan characteristics.

As decision after decision issues from the Reagan White House, building the record of his administration, the president sometimes seems to outsiders to be detached, or even isolated, from the process that produces them.

Reagan's unfamiliarity with major features of some of his own decisions, such as the basing plan he announced for the MX missile last October, has created the impression that he is not on top of issues. This impression is reinforced by his penchant for telling anecdotes rather than analyzing an issue, and by several press conferences where the president seemed unprepared.

On one occasion, after Defense Secretary Caspar W. Weinberger or Secretary of State Alexander M. Haig Jr. differed publicly on a matter of NATO policy, Reagan seemed not to know which was correct. All that deputy press secretary Larry Speakes could contribute, when pressed by reporters, was: "He does know, he just didn't want to tell you."

Even Reagan's senior advisers do not agree among themselves on how much Reagan does know--or on how much he parcels out his decisions to others. They do agree that the level of presidential involvement in the decision-making process varies with the type of decision.

On matters of legislative tactics, Reagan usually defers to those with more experience in Washington, including congressional leaders. Where communicative tactics are involved--the tone of a speech, or whether a particular policy should be announced on television--Reagan listens to his own counsel and believes that he knows more about such matters than his staff.

On a number of complex strategic or foreign policy issues Reagan appears to preside over a delegated presidency. On other policy questions on which Reagan holds strong convictions--the tax cuts, budget reductions and military spending increases, which are the cornerstone of Reaganomics--the president actively resists guidance by his staff.

Some in the Reagan administration believe the president was speaking to his staff more than the Tennessee legislature last week when he paid tribute to Andrew Jackson and declared: "It was Jackson who reminded us that 'one man with courage makes a majority.' "

It is an aphorism that has a special ring if the one man is a president who is rigidly faithful to his past rhetoric in the face of counterarguments.

Increasingly, the president seems to see himself as the last Reaganite. Despite the warnings of top advisers, leading businessmen and Republican congressmen that his entire program could be lost to mounting budget deficits, Reagan has stubbornly refused to recognize that a major problem is building that could overwhelm his administration.

"The problem with Reagan right now is that he doesn't realize he has a problem--or at least the extent of it," said one man who knows Reagan well and asked not to be identified by name. "The 'fairness issue' is hurting him, and he's stubborn about not recognizing it."

Stubborness is an increasingly mentioned Reagan characteristic. Reagan's senior advisers are not hesitant about giving him their opinions, but some have become resigned to seeing their recommendations ignored.

Some say that the president, now 71, is more inflexible than he was a few years ago. Others believe that he became more cognizant of the limited time he has to accomplish his goals after the attempt on his life in which he was badly wounded a year ago. Still others observe that Reagan's abundant supply of optimism seems more plentiful than ever, and that he inevitably believes that everything will turn out for the best.

"It's remarkable," said an aide. "You can show him five charts of decline and he will inevitably find the one blip which shows that the economy is getting better."

Reagan's own spirits remain high. His health is remarkable except that his deafness has increased noticeably. Some advisers have taken to raising their voices when they address him. When he turns on television, the president plays it at a volume that is too loud for others in the room.

The deafness makes it easier for Reagan to tune people out. It isolates him a little more, particularly since he is unwilling to make concessions to it by asking people to repeat themselves when he hasn't heard something clearly.

His advisers have persevered despite their leader's apparent instransigence.

Those who knew him when he was governor of California remember that he once was as firmly opposed to state income tax withholding as he is today to any changes in the tax program. At that time, informed fiscal and political opinion in California was as overwhelmingly in favor of withholding as it is today in favor of a lower federal budget deficit.

Reagan said his "feet were set in concrete" on the withholding issue. He argued that "taxes should hurt" and said that withholding would make it too easy to raise taxes in subsequent years. But when it became apparent that withholding was needed to improve his administration's cash flow position, Reagan stood up at a press conference one day, tapped the microphone and said, "Gentlemen, the sound you hear is the concrete cracking around my feet."

Some of the Californians cite this decision in anticipating a coming compromise with Congress on budget cuts, tax reductions and defense spending. Others acknowledge they are going on faith that history will repeat itself and say that Reagan hasn't yet given any internal signs of flexibility.

Information reaches Reagan through an elaborately designed system of meetings and paper flow. Each day Reagan receives a blizzard of color-coded envelopes prepared for him by presidential assistant Richard G. Darman. Despite his reputation as a "9 to 5" president, aides say that Reagan's overnight reading averages 100 pages of material and that he frequently makes extensive notes in the margin.

One well-informed aide says that Reagan works harder and pays more attention to detail than outsiders realize. But the aide also acknowledges that he frequently uses the material given him selectively to reinforce views he already holds.

National security adviser William P. Clark, who as executive secretary to Reagan as governor in 1967, devised "mini-memos" which strived to present the pros and cons of complicated issues on a single page, has reduced the paper flow to the president while also briefing him more regularly on national security. Reagan, who spent more than two decades as a Hollywood actor and television host, has been especially receptive to filmed briefings on world issues and leaders.

Clark handcarries sensitive papers directly to the president. If a decision is due, he also sends copies to the White House senior trio of top advisers--chief of staff James A. Baker III, deputy chief of staff Michael K. Deaver and counselor Edwin Meese III.

One of the increasingly important meetings in the White House is a Monday "update" luncheon that Deaver introduced partly to keep him more actively aware of issues.

For each meeting, staff members coordinated and directed by Darman and Craig L. Fuller, assemble a gray loose-leaf book with blue tabs for each participant. Each luncheon includes a series of brief presentations: communications director David R. Gergen on the media, Clark on national security affairs, Fuller on Cabinet matters, Ken Duberstein on legislative affairs and Deaver on the president's longer-range schedule.

Reagan is an active participant in the discussions, according to several advisers, and two or three assignments for the staff come out of most luncheons. For example, one recent Monday the president said he wanted to explore ways of drawing more attention to the task force he named to study how to better manage the federal government.

This luncheon is one of the larger meetings Reagan regularly attends. The full Cabinet, which met four times during Reagan's first week in office, meets rarely now. Even the subgroups, called Cabinet councils, which provide smaller, more manageable forums for discussion, do not meet often with the president. Most of his working meetings with advisers are quite small and are dominated by six members of his staff: the Trio of Meese, Baker and Deaver plus Clark, Darman and Fuller. Meetings are deliberately kept small, sometimes restricted to only one or two advisers.

Reagan's style varies according to the type of meeting. At larger meetings he tends to be judicious and disciplined. In an effort to encourage a wide range of discussion and obtain the genuine opinions of his advisers, Reagan often withholds his own opinion.

In smaller groups he is more freewheeling, making frequent use of off-color anecdotes and spontaneous one-liners.

But even in these smaller groups it is not always clear when Reagan has reached a decision. One adviser compares Reagan to a judge who listens to the arguments and then retires to his chambers to make his decision. Another observes that Reagan is politically intuitive and believes he decides on some matters without fully understanding how he reaches his conclusions.

On policy issues Reagan makes strong ideological arguments that sometimes with difficulty can be countered by factual rebuttals. Aides know that their chances of changing Reagan's mind usually depend on whether they can fit their arguments into the president's philosophical framework.

Some aides believe that Reagan's decision-making is influenced by his experience as a hard-bargaining negotiator for the Screen Actors Guild. Sometimes he tells aides about these long-ago negotiations and draws lessons from them about how to achieve a political compromise.

It is pragmatic references such as these that Reagan advisers have in mind when they anticipate a compromise with Congress. But even those who know Reagan best acknowledge that they are making predictions based on past behavior rather than on the president's present inclinations.

As the administration nears a crossroads, Reagan's course of action remains as much a mystery to his staff as it does to the rest of the country.

"I know what he's done in the past and I would expect him to be realistic about the situation this time," one of the president's veteran aides said last week. "But I will also truthfully tell you that's just a guess. What's going on in his head, I just don't know."