Dr. Paul Kalanithi was at the top of the world, but a startling revelation forced him to confront his mortality and decide what legacy he'd like to leave behind.

At 36, Kalanithi had proven himself to be a fascinating individual — while an undergraduate at Stanford, he majored in both English and biology, then earned master's degrees in English literature and philosophy before attending medical school at Yale. But behind his erudite reputation, Kalanithi had a wicked sense of humor; his wife Lucy recalls to ABC News that when they met as first-year medical students, she noticed he was wearing a fake mustache in his ID photo. In fact, she tells reporters that his quirks led her to develop a crush that eventually resulted in their marriage.
Both Kalanithi and his wife worked at Stanford after graduation, he as a neurosurgeon and she as a physician. Their perfect life shuddered to a halt, however, when Kalanithi was diagnosed with Stage 4 metastatic lung cancer — despite never having smoked — and was told he had maybe a year left to live. Rather than wallow in the diagnosis and dwell on how much time he had (or didn't have) left, he chose to make the remainder of his life more meaningful and planned a future that may or may not include him.

According to ABC7 News, Kalanithi chose to complete his training as a neurosurgeon, and on July 4, 2014, he and his wife welcomed their daughter, Cady, into the world. When reflecting upon making the decision to bring life into the world as Kalanithi's was coming to an end, his wife recalls asking him, "Don’t you think having a baby to say goodbye to will make your death more painful?" To which he responded, "Well, wouldn’t it be great if it did?" She explains that the pain would be an indication of a life filled with profound purpose.

In addition to leaving behind a legacy in the form of his daughter, Kalanithi penned his memoirs, called "When Breath Becomes Air," chronicling his battle with lung cancer. Published posthumously, the book is now a New York Times best-seller and the New York Times reports that his widow found it as "a file on his computer labeled Kalanithi-Ms-20Feb.docx" that she later had published. In a heartbreaking passage of his essay, "Before I Go," which was published in Stanford Medical Magazine and in the memoirs as well, Kalanithi imparts this message to Cady:

"When you come to one of the many moments in life when you must give an account of yourself, provide a ledger of what you have been, and done, and meant to the world, do not, I pray, discount that you filled a dying man’s days with a sated joy, a joy unknown to me in all my prior years, a joy that does not hunger for more and more, but rests, satisfied. In this time, right now, that is an enormous thing."

Kalanithi passed away on March 9, 2015, but his heart-wrenching yet hopeful words live on.