FOR YEARS, doctors told their patients that thin was healthy, and they meant really quite thin. Vast research by the life insurance companies showed beyond doubt, they said, that thin people lived the longest. Despite all that scientific and actuarial exhortation, the American profile continued to show a visible inclination toward pudge. Now the Metropolitan Life Insurance Company has issued a new table telling you that the ideal weight is a little higher than they had previously thought. Less thin people now compete more successfully in the great longevity race than they did a generation ago. It is another example of the tendency in recent years of the ideal standard to adjust itself to the intractable reality.

That tendency is very familiar to economists, who have been having more trouble with intractable reality than physicians generally do. Economists used to tell governments confidently how to maintain full employment. Back in the days when thin meant really thin, full employment meant an unemployment rate of 4 percent, and nobody saw any reason why it should be higher. That, needless to say, was in the 1960s. In the 1970s, the economists, led by those who had the misfortune to be in official positions, retreated defensively to rubbery terms like "high" employment, based on a "natural" rate of unemployment that floated up to 5 percent in the early 1970s and is now variously put anywhere from 6 to 6.5 percent. It's sad to see that number keep sliding upward. But it's the same process. It's the target that's being adjusted to the reality.

There was a time--the same time, come to think of it, in which those earlier height and weight charts appeared--when it seemed clear that the ordained order of the universe could not permit long-term interest rates to go above 4 percent. Anything as high as 6 percent would mean, directly and inevitably, the end of civilization as we have known it. Now the experts, led by those in the administration, predict that if the long-term rates can ever be coaxed down under 10 percent, there will be such a flowering of economic development as the world has hardly seen.

Have some more ice cream.