In the absence of hard proof on the causes of cancer, the question needing to be asked is, how valuable is the soft proof? When a National Academy of Sciences research committee issued its recent 600-page report on the link between certain foods and cancer, the group's chairman, Prof. Clifford Grobstein, an experimental biologist from San Diego, gave the proper answer: "We are exploring a relationship between still largely unknowns."

Cancer and diet are certainly that. The more reputable the cancer researcher, the more likely he will say that his findings are mere teasings of the problem, not solutions.

In this instance, the NAS researchers affirmed what vegetarians, myself included, have been saying all along: eat vegetables, fruits and whole grains --and avoid disasters like bacon, hot dogs and fat-laden meat--to reduce the risk of cancer. But scientists are not writing for the pamphlet rack at the local health-food store. Their findings are put out, first of all, for their peers: refute this, if you can. For the lay public, if this year's study on diet and cancer isn't knocked down by next year's, then current speculations are on the way to becoming the future's probabilities. Certainty is for the next generation, if then.

Because scientists have yet to refute past reliable studies suggesting linkage between nutrition and health, Prof. Grobstein could say with confidence: "The evidence is increasingly impressive that what we eat does affect our chances of getting cancer."

In cancer research, the context for causes is so large that the public is only now coming to understand that the search for cures is merely beginning. Diet is a small part of the context. Researchers have been reporting that people are "cancer susceptible" in three categories: heredity, environmental variables and behavioral traits.

A person might, for example, eat all the platefuls of broccoli, citrus fruit and bulgar that the NAS recommends for the national menu, but still be high-risk for future cancer. His parents died of the disease or perhaps emotionally he is poor at coping with stress. Likewise someone who is genetically predisposed and who binges on meat may never be among the 400,000 people who die annually of cancer. His immune system, in which normal cells successfully fight off cancerous ones, is sufficiently strengthened by his psychological outlook.

The effect of behavioral traits on either the prevention of cancer or its defeat once it is detected has not attracted large numbers of researchers. But it deserves to. As valuable as the NAS study is, the moment may have been reached when it is pointless to keep trying to convince the last holdouts that diet is relevant to cancer. Cancer wards from coast to coast are filled with smokers, but the tobacco lobby looks in on these death rows and guilelessly exonerates cigarettes. It won't be beyond the meat industry to wage a similar campaign on behalf of its health threats.

In the years ahead, the most beneficial research could well involve the links between psychosocial factors and cancer. It is known from studies of patients who had unexpectedly overcome their cancer that certain personality or emotional traits were common. These included the will to fight, confidence in recovery and the ability to energetically combat stress.

At a recent New York Academy of Medicine conference, Dr. Joan Borysenko, speaking on "fear, hope and cancer," said that "there are considerable data that point to an influence of (behavioral) factors in the course of the disease. Hopelessness, helplessness and fear correlate with decreased survival. These emotions are associated with hormonal changes that depress the activity of the immune system. The preservation of hope, the determination to fight the disease and the will to live are correlated with enhanced survival and improved quality of life."

Attitude therapy programs are springing up in all parts of the country. If directed by competent therapists, they have promise of serving cancer patients well. It could be that we are about to learn as much about the relationship between a person's attitude and his immune system as we have already discovered about cancer and what he's eating for dinner.