A year ago, I lost my best friend, Oliver Sacks. For many years, each week, Oliver and I would cruise north on the West Side bike path at sunrise. Alone, our bicycles a few inches apart, we spoke about everything and anything, but mostly about interesting patients, natural history, and food. His voice was soft, and I struggled to hear his words. But his volume and pedalling cadence always accelerated when the massive TRUMP PLACE buildings appeared to our right. He detested the giant protuberances that unpleasantly punctuated the view from our bike seats, and often cursed them.

Instead, he looked forward to passing by the Seventy-ninth Street Boat Basin, which reminded him of his City Island days. There, he had a housekeeper who, once a week, would make a beef stew for him and divide it into seven daily portions. One day, when the portions began to decline in size, Oliver asked, “Did the price of beef go up? I will give you more.” His housekeeper sheepishly admitted to pilfering some stew; she could not afford it for herself. “Then I will give you money for eight pounds instead of four, and you keep half.”

We would climb the small hill into Riverside Park’s Ninety-first Street garden for a water stop, and Oliver would become absorbed by a crocus, columbine, hyacinth, or tulip. A stray dandelion once launched a discourse on their unfair label as weeds, the potential diuretic effect of their leaves, their definite edibility (he popped it in his mouth, stem and all), the plant’s name (the coarsely toothed leaves resembled lions' teeth, leading the French to call it dent de lion), and the paradoxical fecundity of these asexual plants. Almost every living eukaryote—organisms with complex cells, from algae and fungi to plants and animals—reproduces sexually, at least some of the time. But certain dandelion species only reproduce asexually. Oliver predicted their “imminent” extinction, at least in geological time, since “only bdelloid rotifers survived tens of million of years living the sexless life.” It was one of the rare times I had something to add. John Maynard Smith, I told him, considered the bdelloid’s successful asexuality “an evolutionary scandal.”

“Very good,” Oliver agreed, with his broad, mischievous smile.

Further north, we came upon the gorgeous wilderness above 140th Street, where a native countryside emerged and we imagined the island before man. “Mastodons roamed Manhattan once,” he reminisced, as if he had seen them as a boy—and then the George Washington Bridge came into sight. Reality returned and we headed home.

Oliver loved movement. He said his best conversations with Robin Williams were on Lake Tahoe, as he swam the backstroke while Robin kayaked alongside. Their eyes never met, but the words flowed. The comfortable silence that friends share rarely lasted long on our bike rides, as Oliver unfurled precious facts and anecdotes. Eating fireflies, he told me, could be lethal, on account of a toxin; although the only confirmed deaths were in lizards, he made me promise never to eat more than two.

As a young boy, Charles Darwin’s son Leonard earnestly asked a friend, “Where does your father do his barnacles?,” thinking that all fathers spent their days peering at barnacles under a microscope. This anecdote delighted Oliver beyond words, partly for the comedy, but mainly because he understood why Darwin would spend eight years with Cirripedia; Oliver was equally obsessed with invertebrates. When he learned how intelligent octopuses were, he gave up eating them, and led others, like me, to do the same. Oliver had little appetite for the political arena and never cast a ballot in an American election. He voted with his pen and his palate.

Oliver’s greatest gift was sensitivity—seeing, feeling, and sketching what the rest of us had never even noticed. I referred many patients to Oliver. He spent two, three, or more hours on the initial visit. Some did not know “who he was,” but, after their consultation, all knew how special he was, all wanted more. His notes overflowed with nuances of their lives; he captured their voices and gained tender and brilliant insights that had escaped me during a decade of care.

He would have been crushed by the rise of Donald Trump and the electoral success of Brexit. Intolerance and fear-mongering, he knew, are rudders that steer societies in dangerous directions. Oliver knew life from the other side: a gay man in a straight society; a doctor who cared for people, not patients; a finder of strength among the infirm. His moral compass pointed to tolerance and kindness. Nearly a decade ago, departing the Havana airport after a swim trip, he was asked if he might donate some clothing for those in need. He told me that he handed over his entire suitcase, and left with his satchel of books, a journal, a magnifying glass, and a few odds and ends, because someone probably needed the rest of his things more than he did.

As he did in Havana, Oliver left us everything he had to give, a treasure of lessons. Care and have empathy for those who are different or less fortunate. Have fun and love often. Find wonder and beauty. Know gratitude.