Last week, I was gratified to see another pertinent, informative and clearly stated op-ed column by Jessica Mathews {"The Greenhouse Holdout," Nov. 9}. This is the same Jessica Mathews referred to in a defamatory letter {Nov. 8} that called her an "environmentalist Malthusian" who used "the standard doomsday rhetoric."

The writer of that letter -- and thousands of other administration supporters and the president's chief of staff -- need some intensive homework about the impact on our society of environmental debasement and the worldwide tendency toward human overpopulation. They might even begin to understand the implications of a deceptively simple formulation by economist/ecologist Kenneth Boulding: "Ecological theory simply says everything depends upon everything else."

The letter writer's reference to "Malthusian" is simplistic in relation to today's population studies. Malthus wrote his "Essay on the Principle of Population" at the end of the 18th century. His emphasis upon food availability as the prime factor in control of population growth is, except in isolated instances such as Ethiopia, only one possible factor in today's industrialized, high-tech, high-energy utilization and polluted world. We live in a world whose products make possible a population growth beyond the natural carrying capacity of the environment. The engineered supply of a diminishing water resource to the American southwestern desert states is an example.

Jessica Mathews, because she is a committed 20th century scientist, writes about this contemporary world as a well-informed, articulate communicator and is most certainly no Malthusian. I am delighted that we have a Jessica Mathews to alert us to the vulnerability of our Spaceship Earth.

ARNOLD R. CHALFANT Winchester, Va.