The president's penchant for not confusing himself with the facts is well known. In the case of the Law of the Sea treaty, he seems to have gone to exceptional lengths.

Washington Post Staff Writer Lou Cannon has passed on what President Reagan said at the National Security Council meeting at which he decided not to sign a treaty that covers two-thirds of the world's surface and took 14 years to negotiate:

"We're policed and patrolled on land, and there is so much regulation that I kind of thought that when you go out on the high seas you can do what you want," he said, proud apparently that no data clogged his mind.

To preserve his ignorance, Reagan never consulted the man who could have set him straight. Elliot L. Richardson, appointed by Jimmy Carter as a representative to the Law of the Sea Conference, spent four years negotiating the draft convention.

Richardson, a notoriously able and proper Bostonian, indispensable to Richard M. Nixon until 1974 when he refused to fire Watergate Special Prosecutor Archibald Cox, never got closer to the Reagan throne than Counselor Edwin Meese III, whom he saw a year ago.

Meese, whose approach to complex foreign policy questions is acceptably ideological, presided over a fatal meeting at which the decision was effectively made to scuttle the sea treaty.

Ironically, initial impetus for the Law of the Sea came from Reagan's favorite government department, Defense. The Joint Chiefs of Staff were anxious to secure international agreements for safe passage of submarines through straits and coastal waters.

But, as the president indicated, the very idea of anything being put beyond the grasp of U.S. business interests is anathema. Almost immediately after taking office, he ordered a review and fired leaders of the U.S. negotiating team. They were suspected of having been "seduced" by long exposure to subversive elements in the United Nations, under whose auspices the Law of the Sea Conference had been laboring since 1966.

The U.N. view of the sea as "the common heritage of mankind" was dangerous nonsense to Reagan. The notion that we would compete with uppity Third World countries for rights to mine the seabeds was an affront. Reagan felt that proposed establishment of an International Seabed Authority was the most unconscionable aspect of the whole contraption.

Some in inner White House councils advocated outright U.S. withdrawal from the conference. They lost, but not really. According to Leigh Ratiner, a former lobbyist for Kennecott Corp. and Reagan's handpicked choice to bargain for a better mining deal, the Reaganites gave him such rigid instructions that he was practically forbidden to make small concessions that would have led to genuine compromise and advantage. He was under secret orders to bring about a "frontier mining code" for the bounding main.

On April 30, the United States was one of four countries to vote against the Law of the Sea. We have not been as isolated since we cast the world's sole nay vote last year against adoption of the code covering infant formula.

What Reagan ended up doing, in the minds of most of the world, was shooting himself in the foot. He has succeeded in excluding U.S. interests from mining rights he thought he was protecting.

Triumphant treaty opponents have assured U.S. miners that they can ignore the treaty and make "mini-treaties" with their allies.

Not so, Richardson says. If he had been given a chance to explain the realities to the president, he says, he would have told him that there "will be no U.S. undersea mining outside the treaty, except under a foreign flag."

As a practical matter, claims outside the treaty would be subject to litigation in the World Court, making banks leery of loaning the vast sums needed for such enterprises.

Richardson says he would also have explained, had he ever made it to the Oval Office, that the United States cannot expect to boycott the treaty and take for granted navigational safeguards under international law--"We cannot pick and choose among rights and obligations."

Under Reagan, the pro-treaty defense line broke. Pentagon ideologues held that strategic minerals under the seabed constitute "national security" interests. In the "damn the treaty" spirit, Navy Secretary John F. Lehman Jr. said, "We can blast our way through the straits."

Richardson tried to express cautions to national security adviser William P. Clark, who suggested that he deal with a staff member.

This week, Richardson went before a House committee studying the treaty wreckage. Several conservative members questioned his patriotism and intelligence for having participated in an international cabal against U.S. business. He replied tartly that, having been considered fit to serve in four Cabinet posts, he did not regard himself as either stupid or subversive.

He does not think the treaty is sunk forever. Another administration may reconsider.

Who knows? Another president might even ask him about it.