THERE ARE DAYS-probably you've had them, too-when even the Haagen-Dazs coffee ice cream has no flavor. The green things are covered with dust, the air is mephitic, the IRS wants a major feeding, and your tooth aches. If you went to the beach, the sharks would attack-and not without reason, one suspects (which has got to be the worst suspicion of all). In the words of the poet whose name is lost to memory but whose lovely lines remain, "the lizard lieth lurking in the grass." Definitely.

I have a prescription for such days. It may work, but if it doesn't activate your dormant sense of wonder I can only suggest that you go to bed for a day or two. Maybe you'll feel better when you wake up. (Thirty-five percent of patients with rheumatoid arthritis, after all, just get better-no matter what the treatment or lack of it.) The prescription is Dr. Thomas' magical placebo (placebo , from the Latin: I shall please), these delightful "notes of a biology watcher" by our own Montaigne of medicine. They are, at the very least, instructive, and a lot more fun than vitamins, which for us, as Dr. Thomas points out, "have taken the place of prayer."

You may remember Lewis Thomas. He won the National Book Award in 1974 for his first collection of essays, The Lives of a Cell , most of which, like these, first appeared in The New England Journal of Medicine . He began writing at 57 and is now 65. He is president of Memorial Sloan-Hettering Cancer Center in New York. Sometimes he is, as I have said, a Montaigne of medicine; sometimes he is a Woody Allen in a white lab coat.

There ia an excavation pond in Manhattan into which people seemed to have dumped their pet goldfish, which flourish and multiply outrageously. "The ASPCA was called [obviously the pond was on the East Side], and came one afternoon with a rowboat. Nets were used, and fish taken away in new custodial bowls, some to Central Park, others to ASPCA headquarters, to the fish pond . . . An official stated for the press that the owners of the property would be asked to drain the pond by pumping, and then the ASPCA would come back with nets to catch them all.

"You'd think they were rats or roaches, the way people began to talk. Get those goldfish out of the pond, I don't care how you do it. Dynamite, if necessary. But get rid of them. Winter is coming, someone said, and it is deep enough so they'll be swimming around underneath the ice. Get them out."

Now in his Woody Allen persona, Dr. Thomas continues. "I thought I noticed a peculiar sort of fin on the undersurface of two of the fish. Perhaps, it occurs to me now in a rush of exultation, in such a pond as this, with all its chemical possibilities, there are contained some mutagens, and soon there will be schools of mutant goldfish. Give them just a little more time, I thought. And then, with the most typically Manhattan thought I've ever thought, I thought: The ASPCA will come again, next month, with their rowboat and their nets. The proprietor will begin pumping out the pond. The nets will flail, the rowboat will settle, and then the ASPCA officials will give a sudden shout of great dismay. And with a certain amount of splashing and grayish-greenish spray, at all the edges of the pond, up all the banks of ancient New York landfill mud, crawling on their new little feet, out onto the sidewalks, up and down and across the street, into doorways and up the fire escapes, some of them with little suckers on their little feet, up the sides of buildings and into open windows, looking for something, will come the goldfish."

Ah, what a time it was, Dr. Lewis observes. And what a book this is; lucid, idiosyncratic, leaping imaginatively from observed natural events-cloning, the conception fo a human life in a laboratory dish, the paradoxical relationship of the medusa (a jellyfish) and the nudibranch (a sea slug) in which each creature seems both predator and prey-to elegant meditations and fleeting epiphanies, sometimes bizarre (as in the goldfish story), sometimes mystical.

For Dr. Lewis, a scientist by training, is a mystic by inclination, a very rational sort of mystic, appropriate to the times. He contemplates the medusa and the snail and concludes: "Like a vaguely remembered dream, they remind me of the whole earth at once."

He thinks of ants: "Massed together, all touching, exchanging bits of information held in their jaws like memoranda, they become a single animal . . . One thing I'd like to known most of all: when those ants have made the Hill, and are all there, touching and exchanging, and the whole mass begins to behave like a single huge creature, and thinks , what on earth is that thought? And while you're at it, I'd like to know a second thing: when it happens, does any single ant know about it? Does his hair stand on end?"

He ponders language, and asks if there is a scrambling device in the brain that preserves "the delicate center of the mechanism of language against tinkering and meddling, shielding the mind against information with which it has no intention of getting involved." Perhaps there is a need for secrecy where language is involved. "It is conceivable," he writes, "that if we had anything like full, conscious comprehension of what we are doing, our speech would be degraded to a permanent stammer or even into dead silence. It would be impossible to turn out the simplest of sentences, the lovely Wallace Stevens sentence, for example: 'The man replied, Things as they are, are changed upon the blue guitar. '. . . Some people, very quick on their feet, can catch a fleeting glimpse of a thought just at the moment of its disappearance into the scrambler, and poems by people like Stevens are made in this way."

And, one might add, essays like these, in one of which he thinks about thinking and about music. "Music," he writes, "is the effort we make to explain to ourselves how our brains work. We listen to Bach transfixed because this is listening to a human mind. The Art of the Fugue is not a special pattern of thinking, it is not thinking about any particular thing. The spelling out of Bach's name in the great, unfinished layers of fugue at the end is no more than a transient notion, something flashed across the mind. The whole piece is not about thinking about something, it is about thinking. If you want, as an experiment, to hear the whole mind working, all at once, put on The St. Matthew Paasion and turn the volume up all the way. That is the sound of the whole central nervous system of human beings, all at once."

In examining the smallest parts of the world, Dr. Thomas has flashed on the whole and seen it as a pulsating, infinitely complex, harmonious organism rather like the ant hill, rather like The St. Matthew Passion. It's a lovely way to look at it. CAPTION: Illustration, Drawing of a Medusa from the book; Picture, Dr. Lewis Thomas by Helen Marcus; Copyright (c) 1979, Helen Marcus