WHILE MOST DOCTORS are busy writing prescriptions, Dr. Lewis Thomas is writing books. Thomas, president of the Sloan-Kettering Cancer Center, disturbs professional authors--he writes both infrequently and splendidly. His only other book, The Lives of a Cell, won a National Book Award in 1974, and his new book, The Medusa and the Snail continues with more notes of a biology watcher.

In the Medusa, however, while Thomas sustains the fine prose and short-essay style of the previous work, he abandons the quality that made the former special: he doesn't restrict himself entirely to his field, biology. Thomas's talent lies in his ability to notice, describe and comment on man and nature. When he strays from this pattern, he gets in trouble. In essays such as "Notes on Punctuation" he plays too many intellectual games, and his essay on etymology is simply out of place.

Unlike Lives of a Cell, here Thomas writes what he feels like writing, disallowing any overall structure for the book. But taken one at a time, Medusa reveals Thomas as a gifted humorist, moralist, psychiatrist and critic. Thomas has no poses, no axes to grind, and so he remains humane, candid and optimistic.

In "The Wonderful Mistake," when he discusses the invention of the molecule of DNA, Thomas demonstrates his talent:

We would have made one fatal mistake: our molecule would have been perfect. Given enough time, we would have figured out how to do this, nucleotides, enzymes and all, to make flawless, exact copies, but it would never have occurred to us, thinking as we do, that the thing had to be able to make errors.

The capacity to blunder is the real marvel of DNA. Without this attribute, we would still be anaerobic bacteria and there would be no music....

Thomas's crities claim that he is overly optimistic--overawed by natural harmony. Thomas clearly believes in some grand design: he repeatedly suggests that complex systems, if left alone, will run smoothly. He carries this idea of non-intervention to extremes, however. We should cure disease, he says, because germs are "meddlers," interrupting the body's natural harmony. Thomas, in his enthusiasm for simplification, has mistaken the environmental position of these organisms. Disease-producing bacteria should certainly be eliminated but they are as intricate a part of the natural world as the body they attack.

Thomas misjudges again in "The Health-Care System" where he discusses the "transformation of our environment" into "an immense imdustry...in aid of health." We have become obsessed with health, he contends, and there is something unhealthy about this. "We do not seem to be seeking exuberance in living as much as...putting off dying." While our society may have become a giant whirlpool treatment concerned with collective physiotherapy," Thomas's analysis of our motivation is missing something. He correctly states that we jog and diet because we fear dying, but we may also think we're helping ourselves.

Quibbling with Thomas's conclusions is probably what he wants. He doesn't look to persuade us; undogmatic and exuberant, Thomas wants only that we stand with him, as biology watchers, and wonder about the mysteries of nature.