When Breath Becomes Air is a memoir written by the late neuroscientist Paul Kalanithi. Kalanithi died with terminal cancer at the age of 37.
The main message that Kalanithi gives us in the book is how we can approach death and how at the same time we need to live a meaningful life, as he states:
A few years later, I hadn’t thought much more about a career but had nearly completed degrees in English literature and human biology. I was driven less by achievement than by trying to understand, in earnest: What makes human life meaningful? I still felt literature provided the best account of the life of the mind, while neuroscience laid down the most elegant rules of the brain. Meaning, while a slippery concept, seemed inextricable from human relationships and moral values.
Each sentence of the book is beautifully written: it will make you laugh, it will make you cry…but the fact is Paul Kalanithi was an extraordinary writer: A neurosurgeon who loved and understood words, who loved treating his patients, who went on a quest to understand the meaning of life.
Paul Kalanithi studied literature and biology at Stanford and studied Philosophy at Cambrige University. He finished his degree at Yale School of Medicine.
I knew medicine only by its absence—specifically, the absence of a father growing up, one who went to work before dawn and returned in the dark to a plate of reheated dinner.
Paul Kalanithi
It’s very easy to be number one: find the guy who is number one, and score one point higher than he does.
Paul Kalanithi 
Only after living out in the desert for a while did we realize that some country facts, like the existence of the jackalope, had been deliberately created to confuse city folk and amuse the locals.
Paul Kalanithi
She made me read 1984 when I was ten years old; I was scandalized by the sex, but it also instilled in me a deep love of, and care for, language.
Paul Kalanithi
Though we had free will, we were also biological organisms—the brain was an organ, subject to all the laws of physics, too! Literature provided a rich account of human meaning; the brain, then, was the machinery that somehow enabled it.
Paul Kalanithi
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.
Paul Kalanithi
Only later would I realize that our trip had added a new dimension to my understanding of the fact that brains give rise to our ability to form relationships and make life meaningful. Sometimes, they break.
Paul Kalanithi 
Openness to human relationality doe snot mean revealing grand truths from the apse; it means meeting patients where they are, in the narthex or nave, and bringing them as far as you can.
Paul Kalanithi
Being with patients in these moments certainly had its emotional cost, but it also had its rewards. I don’t think I ever spent a minute of any day wondering why I did this work, or whether it was worth it.
Paul Kalanithi
If boredom is, as Heidegger argued, the awareness of time passing, then surgery felt like the opposite: the intense focus made the arms of the clock seem arbitrarily placed.
Paul Kalanithi
Human knowledge is never contained in one person. It grows from the relationships we create between each other and the world, and still it is never complete.
Paul Kalanithi 
Maybe, in the absence of any certainty, we should just assume that we’re going to live a long time. Maybe that’s the only way forward.
Paul Kalanithi
Graham Greene once said that life was lived in the first twenty years and the remainder was just reflection. So what tense am I living in now?
Paul Kalanithi 
Most ambitions are either achieved or abandoned; either way, they belong to the past.
Paul Kalanithi
Yet the paradox is that scientific methodology is the product of human hands and thus cannot reach some permanent truth. We build scientific theories to organize and manipulate the world, to reduce phenomena into manageable units.
Paul Kalanithi
Science is based on reproducibility and manufactured objectivity. As strong as that makes its ability to generate claims about matter and energy, it also makes scientific knowledge inapplicable to the existential, visceral nature of human life, which is unique and subjective and unpredictable.
Paul Kalanithi
Science may provide the most useful way to organize empirical, reproducible data, but its power to do so is predicated on its inability to grasp the most central aspects of human life: hope, fear, love, hate, beauty, envy, honor, weakness, striving, suffering, virtue.
Paul Kalanithi