THE RIVER OF CONSCIOUSNESS. By Oliver Sacks. Knopf. 237 pages. $27.

Oliver Sacks will be missed.

But our regret at the loss of so remarkable an intellect is salved by the wonderful writing he left behind, a bequest to those with open minds and unquenchable curiosity.

Two weeks before his death in 2015, the gifted neurologist and author outlined the contents of this collection of essays, a posthumously published coda to a distinguished career. “The River of Consciousness” again exhibits Sacks' wide-ranging interests, his profound appreciation of the human animal and the grace and clarity of his prose.

Among the many observations and interpretations herein, Sacks explores the nature and constructs of memory, the varied components of the creative self, the punctuated quality of visual perception and consciousness as a process (not a thing) — all in accessible, thought-provoking language.

Sacks opens with billets-doux to two of his heroes, Charles Darwin and Sigmund Freud.

Sacks corrects our dominant image of Darwin as an evolutionary biologist, the author of “On the Origin of Species” (1859), acquainting us with Darwin's six books and 70 papers on the botanical realm, ground-breaking works that provided far more evidence for natural selection than did his most famous book.

Though botany, arguably the first evolutionary science, was Darwin's first love, all his work in the field came after the publication of “Origin,” much of it accomplished while he was an invalid, beset with a mysterious illness that plagued him for 40 years. For Darwin, also an accomplished geologist, “natural beauty was not just a matter of aesthetics,” Sacks writes. “It always reflected function and adaptation at work.”

In celebrating his career, Sacks broadens our understanding of just how significant a scientist and investigator Darwin truly was.

Sacks performs a similar service for Freud, so often vilified in the 1970s for his psychoanalytical ideas and theories. Often politically motivated, these attacks failed to regard his ideas as what they were: insights on a continuum of discovery, correction and change.

Few recall that Freud first made his mark as a neurologist and anatomist (1876-1896), the first researcher to grasp that the nerve cell body and its processes “constituted the basic building blocks and signaling units of the nervous system,” and making significant contributions to the study of infantile paralysis.

The nature and malleability of memory fascinated Freud all his life. No less so did it captivate Sacks, as did the complex processes involved in perception, memory and recall. His observations on the fallibility of memory are especially intriguing.

Drawing on his own medical practice, as he does throughout, Sacks comments on the transfer of memories: “It is startling to realize ... that some of our most cherished memories may never have happened — or may have happened to someone else,” despite our conviction or sense of actual lived experience or “reality.” It is here that the difference between “historical truth” and “narrative truth” is so often revealed.

The wonder, he says, is that “aberrations of the gross sort are relatively rare and that for the most part our memories are so solid and reliable.” Yet memory “arises not only from experience but from the intercourse of many minds.”

Closely aligned with Sacks' disquisition on memory is his study of visual perception, a gateway to memory. He concludes that “we are incapable of being passive, wholly impartial observers. Every perception, every scene, is shaped by us, whether we intend it or know it, or not. We are the directors of the film we are making — but we are its subjects too: every frame, every moment, is us, is ours.”

Because he lauds all who deserve recognition for their discoveries and breakthroughs, one forgives Sacks' too-frequent plugs for his own books, papers and monographs. Apart from the scientists whose work he details, Sacks also invokes Samuel Taylor Coleridge, Williams James, Mark Twain, Helen Keller, Susan Sontag and the actor who played him (with a name change) in the film “Awakenings,” Robin Williams.

Perhaps Sacks' most arresting chapter is on creativity. He defines it as that state “when ideas seem to organize themselves into a swift, tightly woven flow, with a feeling of gorgeous clarity and meaning emerging — seems to me physiologically distinctive,” featuring “unusual and widespread (mental) activity with innumerable connections and synchronizations occurring.”

He reveled in his own creative energies: “At such times, when I am writing, thoughts seem to organize themselves in spontaneous succession and to instantly appropriate words. I feel I can bypass or transcend much of my own personality, my neuroses. It is at once not me and the innermost part of me, certainly the best part of me.”

Philosophy aside, will we ever fully decipher the code of consciousness? Sacks had his doubts.

“We cannot begin to catch the density, the multifariousness of it all, the superimposed and mutually influencing layers of the stream of consciousness as it flows, constantly changing, through the mind. Even the highest powers of art ... can only convey the faintest intimation of what human consciousness is really like.”

Reviewer Bill Thompson is a freelance writer and editor based in Charleston.