LAST night I dreamed about mercury — huge, shining globules of quicksilver rising and falling. Mercury is element number 80, and my dream is a reminder that on Tuesday, I will be 80 myself.
Elements and birthdays have been intertwined for me since boyhood, when I learned about atomic numbers. At 11, I could say “I am sodium” (Element 11), and now at 79, I am gold. A few years ago, when I gave a friend a bottle of mercury for his 80th birthday — a special bottle that could neither leak nor break — he gave me a peculiar look, but later sent me a charming letter in which he joked, “I take a little every morning for my health.”
      
Eighty! I can hardly believe it. I often feel that life is about to begin, only to realize it is almost over. My mother was the 16th of 18 children; I was the youngest of her four sons, and almost the youngest of the vast cousinhood on her side of the family. I was always the youngest boy in my class at high school. I have retained this feeling of being the youngest, even though now I am almost the oldest person I know.
      
I thought I would die at 41, when I had a bad fall and broke a leg while mountaineering alone. I splinted the leg as best I could and started to lever myself down the mountain, clumsily, with my arms. In the long hours that followed, I was assailed by memories, both good and bad. Most were in a mode of gratitude — gratitude for what I had been given by others, gratitude, too, that I had been able to give something back. “Awakenings” had been published the previous year.
      
At nearly 80, with a scattering of medical and surgical problems, none disabling, I feel glad to be alive — “I’m glad I’m not dead!” sometimes bursts out of me when the weather is perfect. (This is in contrast to a story I heard from a friend who, walking with Samuel Beckett in Paris on a perfect spring morning, said to him, “Doesn’t a day like this make you glad to be alive?” to which Beckett answered, “I wouldn’t go as far as that.”) I am grateful that I have experienced many things — some wonderful, some horrible — and that I have been able to write a dozen books, to receive innumerable letters from friends, colleagues and readers, and to enjoy what Nathaniel Hawthorne called “an intercourse with the world.”
      
I am sorry I have wasted (and still waste) so much time; I am sorry to be as agonizingly shy at 80 as I was at 20; I am sorry that I speak no languages but my mother tongue and that I have not traveled or experienced other cultures as widely as I should have done.
I feel I should be trying to complete my life, whatever “completing a life” means. Some of my patients in their 90s or 100s say nunc dimittis — “I have had a full life, and now I am ready to go.” For some of them, this means going to heaven — it is always heaven rather than hell, though Samuel Johnson and James Boswell both quaked at the thought of going to hell and got furious with David Hume, who entertained no such beliefs. I have no belief in (or desire for) any post-mortem existence, other than in the memories of friends and the hope that some of my books may still “speak” to people after my death.
      
W. H. Auden often told me he thought he would live to 80 and then “bugger off” (he lived only to 67). Though it is 40 years since his death, I often dream of him, and of my parents and of former patients — all long gone but loved and important in my life.
      
At 80, the specter of dementia or stroke looms. A third of one’s contemporaries are dead, and many more, with profound mental or physical damage, are trapped in a tragic and minimal existence. At 80 the marks of decay are all too visible. One’s reactions are a little slower, names more frequently elude one, and one’s energies must be husbanded, but even so, one may often feel full of energy and life and not at all “old.” Perhaps, with luck, I will make it, more or less intact, for another few years and be granted the liberty to continue to love and work, the two most important things, Freud insisted, in life.
      
When my time comes, I hope I can die in harness, as Francis Crick did. When he was told that his colon cancer had returned, at first he said nothing; he simply looked into the distance for a minute and then resumed his previous train of thought. When pressed about his diagnosis a few weeks later, he said, “Whatever has a beginning must have an ending.” When he died, at 88, he was still fully engaged in his most creative work.
My father, who lived to 94, often said that the 80s had been one of the most enjoyable decades of his life. He felt, as I begin to feel, not a shrinking but an enlargement of mental life and perspective. One has had a long experience of life, not only one’s own life, but others’, too. One has seen triumphs and tragedies, booms and busts, revolutions and wars, great achievements and deep ambiguities, too. One has seen grand theories rise, only to be toppled by stubborn facts. One is more conscious of transience and, perhaps, of beauty. At 80, one can take a long view and have a vivid, lived sense of history not possible at an earlier age. I can imagine, feel in my bones, what a century is like, which I could not do when I was 40 or 60. I do not think of old age as an ever grimmer time that one must somehow endure and make the best of, but as a time of leisure and freedom, freed from the factitious urgencies of earlier days, free to explore whatever I wish, and to bind the thoughts and feelings of a lifetime together.
I am looking forward to being 80.
      
Oliver Sacks is a professor of neurology at the N.Y.U. School of Medicine and the author, most recently, of “Hallucinations".