A director of the Sloan-Kettering Cancer Center in New York said a few years ago that the greatest fear for 65 percent of Americans "is cancer -- not war, not economic destruction, not famine, not even death. Scientists are not prepared to deal with that kind of force, that emotionalism."

Thanks to Ronald Reagan, the scientists and the rest of us are now a bit better prepared. From his hospital bed, Reagan's gift to the nation has been to put cancer in perspective. He has allowed his surgeons and pathologists to talk publicly about his case and his chances for survival. It would have been that way had Reagan been operated on for heart disease, or a slipped disc, or a broken leg. So why not with cancer?

Reagan, without overplaying the natural breeziness by which he often downplays the somber, has come forward with the ideal attitude: It projects him as a cancer patient, not a cancer victim.

As the country accustoms itself to Reagan's medical vulnerability, the president has benefited by the ex- periences of other national politicians who saw that cancer defeatism was as much to be fought as the cancer itself.

In 1983, Paul Tsongas decided he would rather deal with his illness as a private citizen than as a U.S. senator from Massachusetts. At 42, Tsongas learned that the lump in his groin was not a hernia but an enlarged lymph node. It was diagnosed as an indolent nodular lymphoma.

In "Heading Home," Tsongas wrote that hanging around Washington and being "the senator with cancer" was no kind of future he wanted for himself or his family. He estimated that he had 10 years to live. He chose to go home to Lowell. Ten full years with his wife and three children would be better than 10 political years with his colleagues and career. That was a choice of genuine manliness.

Reflecting on his illness, Tsongas wrote of "negotiating with the reality that we are all going to die. We all had a date, and on the next day the sun would still be shining somewhere, the rain would still be falling somewhere. . . . The earth was timeless, not those who inhabit it. What had to be done had to be done now."

Before Tsongas, Hubert Humphrey showed that cancer could not destroy the vital signs of his personal courage. Humphrey, like Reagan, was an extrovert. He used laughter as a defense. "I try not to think about my illness," he said after being told of his cancer. "So what is my theory, my philosophy? Live each day, do what you want to with whatever strength you have. . . . As I told those doctors the other day, 'Listen, if you let me die, it's going to cost you millions of dollars 'cause I can get you more for your cancer-research projects than you'd ever believe possible. Keep me alive.' So I joke with them about it."

Among Washington's politicians, few were braver in facing their cancer than Philip Hart, the Michigan senator who died in 1976. A year before his death, he gave an interview that put his own feelings on the record. With cancer, he said, death "is something that not only is on schedule but is in motion. And you do review the bidding and test the faith. I think now I'm prepared."

In the survival lottery, no one knows with any exactness what effect a person's mental state has on his disease. Within cancer research, the debate is between those who argue that no evidence is present to prove that a positive attitude can have curative effects and those who are saying: Wait a minute. There may not be hard proof but the soft proof looks good.

An editorial in the June 13 New England Journal of Medicine kisses off the benefits of a positive attitude: "The literature contains very few scientifically sound studies of the relation, if there is one, between mental state and disease."

The other side of the argument is more compelling: Unassailable evidence may not be on hand, but enough researchers have been exploring the links between personality and immune systems to conclude that strong possibilities exist that personality does play a role. What kind is not yet known. It isn't quackery to say that a healthy mind and a sick body affect each other. It's what first-year medical interns understand when they try to cheer up the patients on the ward.

What Betty Ford did for breast cancer, Ronald Reagan now appears to be doing for bowel cancer: putting the hush-hush days behind us. The American Board of Colon and Rectal Surgery has reported that colorectal surgeons -- who deal with the nation's second leading cancer killer -- are in demand as never before by people wanting checkups. Another stigma fades.