SANTA BARBARA, CALIF. -- One of the most reassuring qualities of President Reagan, who hasn't otherwise been particularly reassuring lately, is that he never lets the burdens of office get in the way of his recreation.

In a society where workaholism is seen as a necessary disease, Reagan remains serenely unafflicted. He likes to ride and do chores on his 688-acre mountaintop ranch. He does not pretend that he is boning up on the intricacies of arms control or instruct spokesmen to misrepresent his ranch time as a "working vacation," a fiction employed by several predecessors. As Reagan put it last March, "It's true hard work never killed anybody, but I figure, why take the chance?"

Reagan was joking, of course. "Hard work" means different things to different people, and Reagan works very hard, if physical energy is the principal measurement. At 76, he is stronger and more active than most of the younger people who work for him, not to mention the reporters of all ages who share in the good life of Santa Barbara while Reagan rides the range at Rancho del Cielo.

What Reagan lacks is a willingness to expend intellectual energy unless it is an absolute requisite of his job. He will work hard enough when presented with a speech draft on deadline, usually in reaction to the ideas of others. He has an almost total lack of curiosity and no interest at all in solving a problem for its own sake. Aides cannot remember the last time he read a book.

Perhaps these harsh judgments are the products of one too many White House briefings on the joys of clearing brush or chopping firewood at the Reagan ranch. No doubt these opinions also are grist for the mill of adversaries who have made a career of underestimating Reagan's brainpower and are even now prematurely proclaiming him a lame duck.

But if we are to learn useful lessons from the painful experiences of the Reagan presidency, it is important to reexamine the strengths and weaknesses of the president. In the twilight of his power, he remains a figure of appealing strength, patriotism and personal charm, secure in his convictions as well as his ignorance. At home on Rancho del Cielo, he seems the quintessential American cowboy ready to ride into town to say what he means and set things right.

What is missing from the picture is any reflective quality. Reagan appears incapable of a critical assessment of what has gone wrong in the country during his presidency. He vaguely alludes to "mistakes" in the Iran-contra affair but reduces a scandal entirely of the administration's making to a struggle for primacy between the executive and legislative branches. What he really thinks of the affair is reflected in his historically inaccurate comment to Time magazine's Hugh Sidey that Democratic Congresses have never investigated Democratic presidents.

In that same interview, one in which Reagan seemed to value what Sidey called his "career as a kid" more than his careers as actor and politician, the president also stated that economic advisers had told him before the 1980 election that "there was no way that we we're going to in a few years be able to balance the budget." Readers supplied with retentive memories or access to researchers might be tempted to wonder if Reagan was telling the truth when he said, on Oct. 28, 1980, "I have submitted a plan that can provide for a balanced budget by 1983, if not earlier."

I don't think that Reagan was lying, when he said these words, certainly not in the sense that he was consciously misleading the American people. Reagan believes whatever he wants to believe, as even some of his friends have sadly acknowledged. He does not seem to know that the budgets he has sent to Congress have failed to provide revenue for programs he thinks necessary. Instead, Reagan rationalizes what has happened and has now convinced himself that he never believed a balanced budget was actually possible.

It is refreshing to have a president who believes in honest physical labor and is not afraid to work or to take vacations. Unfortunately, these qualities are no substitute for a leader who thinks about what he is saying and is capable of understanding why his presidency went awry.

Reaganism of the Week: Asked in the Sidey interview what he would do after his term was over, the president said, "A lot more ranching than I get to do now . . . . But I have a hunch I will be back in the mashed-potato circuit, campaigning for things I believe in and people I believe in."