Jessica Mathews wrote that the White House (in the person of John Sununu) will not join European and some other nations in putting a cap on CO-2 emissions. {"The Greenhouse Holdout," Nov. 9} If we commit to the cap, we will have to do it, and we don't know how. In fact, neither do the other nations.

Dr. Mathews's basic error: "But how accurate the studies are -- whether Germany cuts its emissions by 25 percent or half of that -- is not what matters. The goals ... shape policy." But the numbers do matter. Goals, if they are unrealistic or unreachable, can dictate policy choices that could destroy the hopes of society. Dr. Mathews may be right that these are not "phony goals." But the policies necessary to reach them could stall industrial and economic progress, and the world is filled with examples of where poverty equals environmental disaster.

If great profits are to come from efficiency and alternative energy sources, there are no eager investors, either government or private.

Sweden committed to a cap on CO-2 emissions in 1988. But Swedes also voted for no more hydroelectric dams and for a phase-out of nuclear power. Now they admit that something has to give. Being realistic about goals does matter. A. DAVID ROSSIN Los Altos, Calif.

The writer was assistant secretary for nuclear energy, Department of Energy from 1986 to 1987.