MOST AMERICANS find it very difficult to believe that this country, over the past decade, has grown substantially richer. The very suggestion runs counter to years of lamentation by politicians and economists alleging a long-term decline in standards of living. Personal income per capita last year was higher than ever before--higher even after inflation, higher even after income taxes. But people seem to mistrust that kind of statistic and ask, reasonably enough, what the money buys. The record of the 1970s is a great deal better than the public pessimism of the period indicates.

The best single measure of the quality of physical life is simply longevity. At the beginning of the decade, life expectancy at birth for an American was 70.8 years; in 1980 it was 73.6 years--an enormous increase, and the most rapid advance since the 1940s. This reflects not only the state of medical care but diet and the broadest definitions of living and working conditions.

Another basic factor in the standard of living is housing. In the 1970s, the number of housing units rose more than twice as fast as the population. Crowding decreased. Housing conditions in this country have for some time been good enough that the numbers of homes without running water or electricity is no longer significant. But, particularly in the present weather, you will probably agree that air conditioning is more than a frill. In 1970, slightly over one-third of all American homes enjoyed air conditioning; currently, well over half of them do.

Access to education is crucial to most people's idea of the standard of living. A decade ago, one out of nine adult Americans had had four years of college; by 1980, it was nearly one out of six. Total college and university enrollment at the beginning of the decade was 8.6 million; at the end of the decade, it was 12.2 million. A couple of qualifications: by 1980 the proportion of part-time students was much higher, and some of the most recent increase was the result of an expansion of federal aid that has now ended. But it's clear that the level of education in American society rose substantially in the 1970s.

The list could be continued at some length. The challenge to the historians of these years will be to explain why so many Americans thought that the national economy and living standards were in decline when, by most measures, things were going pretty well. No doubt much of the answer lies in the fears generated by the waves of inflation and rising unemployment.

But there is more at stake here than detached historical interest. The Reagan administration came to office burning with the conviction that it had to deal with a deeply embedded, long-term decline in American economic performance. That was a mirage. The threat was inflation. It had got badly out of control in the Carter years, and Americans had used it to keep the standard of living rising a little faster than the country could afford. But the perception of fundamental, structural economic decline was wrong--which entitles you to doubt that the present administration is right in its prescriptions for fundamental, structural economic reforms.