Newspapers exist primarily to inform. But the first and finest columnist (Samuel Johnson) rightly insisted that men more frequently require to be reminded than informed. So let us now be reminded of Jack Swigert's gifts to his country. He gave examples of bravery when he did not die, and when he did.

Some public persons take too much to heart Longfellow's announcement that life is real, life is earnest. They are oppressively somber, their backs bent, figuratively speaking, beneath the weight of Responsibility. Swigert was not like that. He had life in perspective, perhaps because he had felt the breath of death.

The first time was with the world watching. To be precise, the world was listening to the laconic space-speak between Houston and Apollo 13. It was April 13, 1970--a great day for the superstitious.

Suddenly, crackling across 206,000 miles came words more memorable to those who heard them than "the Eagle has landed." The words, spoken in a tone of tenseness not hitherto heard from space, were: "We've got a problem." Suddenly, the space program stopped being automatic and antiseptic and became a matter of desperate improvisation to save three men outbound at 2,200 mph.

An oxygen tank had exploded, damaging the command capsule's electricity, computer, water and fuel cells. Instead of heading for a moon landing, astronauts Swigert, James Lovell and Fred Haise were in danger of dying as no one ever had, hurtling into deep space. But with superb help from Houston, they launched themselves into a maneuver that whirled them around the moon and homeward.

Last summer, when Swigert was running for Congress, a small malignancy was removed from his nasal passage. When a physical examination in August left him with mysterious back pains, he underwent extensive tests, which revealed cancer in his bone marrow. By then it was too late for Colorado Republicans to field another candidate so, like a spacecraft locked in orbit, he continued, adding chemotherapy, with its debilitating effects, to the wear-and-tear of campaigning. He lost some hair and frequently looked ravaged, but he gave an imperishable example to a national constituency--the 3 million Americans who have cancer and are summoning the physical and emotional stamina to carry on.

There was reason to hope, if not to expect, that the therapy would arrest the disease. In 1982, as in 1970, Swigert acted on the principle that however long the odds, you are not beaten until you concede. But shortly before Christmas, pain drove him into a Washington hospital.

There, to the end, he was what he had been all his life: in control. Surrounded by and attached to the paraphernalia of high-technology medicine, he monitored his condition. He had a mind disciplined by close encounters with danger (he survived a plane crash while an Air Force fighter pilot in Korea) and eyes trained to extrapolate reality from electronic impulses on gauges and screens.

At one point he told a doctor that he thought one lung was filling and that he should have an X-ray. The doctor disagreed, so Swigert bet him a steak dinner. Before 24 hours had passed, Swigert had died at age 51, a week before he was to have been sworn in as a congressman.

Heroism is not as promiscuously distributed as President Reagan said in his inaugural address, when he baptized us all heroes. But there are humble, hidden forms of heroism. A gifted professional basketball player (Spencer Haywood), asked how it felt to be a hero, responded that if you want to see heroism, watch a welfare mother raising a large family.

But public acts of heroism, although not morally grander than private bravery, have special power as models of virtue. The communication of patterns of excellence once was a function of literature. However, the tutelary task once performed by, say, the Bible, Bunyan and Shakespeare is no longer performed by any corpus of great books that shape the community's moral imagination.

Walker Percy writes novels of moral seriousness, but his characters do not make virtue vivid. John Updike is a gifted writer, but his Harry "Rabbit" Angstrom and other residents of Brewer, Pa., are not intended to do for us what Homer's and Dante's creations did for Greece and Italy--supply unifying patterns of excellence. Contemporary literature is doing some serious things, but not that serious thing.

Nations always, but especially in an era of nonheroic or antiheroic literature, need conspicuous real heroes. This nation just lost one.