“There is, in sanest hours, a consciousness, a thought that rises, independent, lifted out from all else, calm, like the stars, shining eternal,” Walt Whitman wrote in contemplating identity and the paradox of the self. Whitman lived in an era before the birth of neuroscience, before psychology as we know it became a robust field of scientific study — before, that is, we began examining more closely whatever it is that we mean by the “self,” only to find that it doesn’t hold up to systematic scrutiny. A century after Whitman, another great poet and great seer of the human experience articulated the terror and the beauty of this elemental fact: “The self is a style of being, continually expanding in a vital process of definition, affirmation, revision, and growth,” Robert Penn Warren wrote in admonishing against the trouble with “finding yourself,” “a process that is the image, we may say, of the life process of a healthy society itself.”

Around the same time, a poet laureate of the life process — the great physician, etymologist, poet, and essayist Lewis Thomas (November 25, 1913–December 3, 1993) — explored the confounding nature of the self with uncommon insight and originality in the title essay of his altogether magnificent 1979 collection The Medusa and the Snail: More Notes of a Biology Watcher (public library).

Thomas writes:

We’ve never been so self-conscious about our selves as we seem to be these days. The popular magazines are filled with advice on things to do with a self: how to find it, identify it, nurture it, protect it, even, for special occasions, weekends, how to lose it transiently. There are instructive books, best sellers on self-realization, self-help, self-development. Groups of self-respecting people pay large fees for three-day sessions together, learning self-awareness. Self-enlightenment can be taught in college electives.

You’d think, to read about it, that we’d only just now discovered selves. Having long suspected that there was something alive in there, running the place, separate from everything else, absolutely individual and independent, we’ve celebrated by giving it a real name. My self.

In a testament to Ursula K. Le Guin’s conviction that “we can’t restructure our society without restructuring the English language,” Thomas traces the etymology of self, folded into which is just about the entire history of the human world:

The original root was se or seu, simply the pronoun of the third person, and most of the descendant words, except “self” itself, were constructed to allude to other, somehow connected people; “sibs” and “gossips,” relatives and close acquaintances, came from seu. Se was also used to indicate something outside or apart, hence words like “separate,” “secret,” and “segregate.” From an extended root swedh it moved into Greek as ethnos, meaning people of one’s own sort, and ethos, meaning the customs of such people. “Ethics” means the behavior of people like one’s self, one’s own ethnics.

Embedded in this evolutionary history of our language is something wholly uncorroborated by the evolutionary history of our biology — the misplaced hubris of exceptionalism. Thomas writes:

We tend to think of our selves as the only wholly unique creations in nature, but it is not so. Uniqueness is so commonplace a property of living things that there is really nothing at all unique about it. A phenomenon can’t be unique and universal at the same time. Even individual, free-swimming bacteria can be viewed as unique entities, distinguishable from each other even when they are the progeny of a single clone.

Thomas points out that creatures large and small exhibit properties that, in their human manifestation, we call individuality — they are, in other words, distinct selves. Single-cell microorganisms swimming in the same water, when examined closely enough, can be distinguished from one another by the way they twirl around their flagellae. Beans carry glycoproteins that serve as self-labels. Coral polyps are endowed with a biological self-consciousness that allows them to recognize other polyps of the same genetic line to fuse with, rejecting polyps of different lines. Fish and mice can tell individuals of their species by their smell. (Decades after Thomas composed this essay, we know that trees also differentiate between and communicate with individual others.) He considers the biological function of the self:

The markers of self, and the sensing mechanisms responsible for detecting such markers, are conventionally regarded as mechanisms for maintaining individuality for its own sake, enabling one kind of creature to defend and protect itself against all the rest. Selfness, seen thus, is for self-preservation.

In real life, though, it doesn’t seem to work this way. The self-marking of invertebrate animals in the sea, who must have perfected the business long before evolution got around to us, was set up in order to permit creatures of one kind to locate others, not for predation but to set up symbiotic households. The anemones who live on the shells of crabs are precisely finicky; so are the crabs. Only a single species of anemone will find its way to only a single species of crab. They sense each other exquisitely, and live together as though made for each other.

Thomas locates the most compelling and sobering illustration of this in two obscure species inhabiting the Bay of Naples, melded into one — a common nudibranch sea slug and the medusa of a tiny jellyfish, permanently affixed to the shell-less snail’s mouth as a vestigial parasite. When marine biologists first discovered the improbable pair and set out to investigate how it formed, they found something astonishing and wholly counter to our basic assumptions about the orientation of a self to an other. Thomas writes:

The attached parasite, although apparently so specialized as to have given up living for itself, can still produce offspring, for they are found in abundance at certain seasons of the year. They drift through the upper waters, grow up nicely and astonishingly, and finally become full-grown, handsome, normal jellyfish. Meanwhile, the snail produces snail larvae, and these too begin to grow normally, but not for long. While still extremely small, they become entrapped in the tentacles of the medusa and then engulfed within the umbrella-shaped body. At first glance, you’d believe the medusae are now the predators, paying back for earlier humiliations, and the snails the prey. But no. Soon the snails, undigested and insatiable, begin to eat, browsing away first at the radial canals, then the borders of the rim, finally the tentacles, until the jellyfish becomes reduced in substance by being eaten while the snail grows correspondingly in size. At the end, the arrangement is back to the first scene, with the full-grown nudibranch basking, and nothing left of the jellyfish except the round, successfully edited parasite, safely affixed to the skin near the mouth.

More than a century after the great naturalist John Muir insisted that “when we try to pick out anything by itself, we find it hitched to everything else in the universe,” Thomas finds in this real-world Aesop fable drawn from the evolutionary record an embodiment of the exquisite interdependence of nature — an elemental awareness of which we so easily and habitually lose sight, and yet an awareness which, when fully apprehended, dissolves the very notion of a discrete self:

It is a confusing tale to sort out, and even more confusing to think about. Both creatures are designed for this encounter, marked as selves so that they can find each other in the waters of the Bay of Naples. The collaboration, if you want to call it that, is entirely specific; it is only this species of medusa and only this kind of nudibranch that can come together and live this way. And, more surprising, they cannot live in any other way; they depend for their survival on each other. They are not really selves, they are specific others.

The thought of these creatures gives me an odd feeling. They do not remind me of anything, really. I’ve never heard of such a cycle before. They are bizarre, that’s it, unique. And at the same time, like a vaguely remembered dream, they remind me of the whole earth at once. I cannot get my mind to stay still and think it through.

The essays in The Medusa and the Snail, which include Thomas’s beautiful meditation on altruism and the scientific poetics of friendship, remain among the finest, most insightful writing I have ever savored. Complement this particular portion with the young Borges on the nonexistence of the self, Ian McEwan on how the cult of selfhood imperils society, philosopher Jacob Needleman on how we become who we are, and neuroscientist Sam Harris on the paradox of free will, then revisit Thomas on our human potential and our cosmic responsibility.