It is sometimes said that existentialism is more of a mood than a philosophy, and that it can be traced back to anguished novelists of the 19th century, and beyond that to Blaise Pascal, who was terrified by the silence of infinite spaces, and beyond that to the soul-searching St. Augustine, and beyond that to the Old Testament’s weary Ecclesiastes and to Job, the man who dared to question the game God was playing with him and was intimidated into submission. To anyone, in short, who has ever felt disgruntled, rebellious or alienated about anything.

But one can go the other way, and narrow the birth of modern existentialism down to a moment near the turn of 1932-33, when three young philosophers were sitting in the Bec-de-Gaz bar on the rue du Montparnasse in Paris, catching up on gossip and drinking the house speciality, apricot cocktails.

The one who later told the story in most detail was Simone de Beauvoir, then around 25 years old and given to watching the world closely through her elegant hooded eyes. She was there with her boyfriend, Jean-Paul Sartre, a round-shouldered 27-year-old with downturned grouper lips, a dented complexion, prominent ears, and eyes that pointed in different directions, for his almost-blind right eye tended to wander outwards in a severe exotropia or misalignment of the gaze. Talking to him could be disorienting for the unwary, but if you forced yourself to stick with the left eye, you would invariably find it watching you with warm intelligence: the eye of a man interested in everything you could tell him.

Sartre and Beauvoir were certainly interested now, because the third person at the table had news for them. This was Sartre’s debonair old school friend Raymond Aron, a fellow graduate of the École normale supérieure. Like the other two, Aron was in Paris for his winter break. But whereas Sartre and Beauvoir had been teaching in the French provinces — Sartre in Le Havre, Beauvoir in Rouen — Aron had been studying in Berlin. He was now telling his friends about a philosophy he had discovered there with the sinuous name of phenomenology — a word so long yet elegantly balanced that, in French as in English, it can make a line of iambic trimeter all by itself.

Aron may have been saying something like this: traditional philosophers often started with abstract axioms or theories, but the German phenomenologists went straight for life as they experienced it, moment to moment. They set aside most of what had kept philosophy going since Plato: puzzles about whether things are real or how we can know anything for certain about them.

Instead, they pointed out that any philosopher who asks these questions is already thrown into a world filled with things — or, at least, filled with the appearances of things, or “phenomena” (from the Greek word meaning “things that appear”). So why not concentrate on the encounter with phenomena and ignore the rest? The old puzzles need not be ruled out forever, but they can be put in brackets, as it were, so that philosophers can deal with more down-to-earth matters.

The phenomenologists’ leading thinker, Edmund Husserl, provided a rallying cry, “To the things themselves!” It meant: don’t waste time on the interpretations that accrue upon things, and especially don’t waste time wondering whether the things are real. Just look at this that’s presenting itself to you, whatever this may be, and describe it as precisely as possible.

Another phenomenologist, Martin Heidegger, added a different spin. Philosophers all through history have wasted their time on secondary questions, he said, while forgetting to ask the one that matters most, the question of Being. What is it for a thing to be? What does it mean to say that you yourself are? Until you ask this, he maintained, you will never get anywhere. Again, he recommended the phenomenological method: disregard intellectual clutter, pay attention to things and let them reveal themselves to you.

“You see, mon petit camarade,” said Aron to Sartre — “my little comrade,” his pet name for him since their school days — “if you are a phenomenologist, you can talk about this cocktail and make philosophy out of it!”

Beauvoir wrote that Sartre turned pale on hearing this. She made it sound more dramatic by implying that they had never heard of phenomenology at all. In truth, they had tried to read a little Heidegger. A translation of his lecture “What Is Metaphysics?” had appeared in the same issue of the journal Bifur as an early Sartre essay in 1931. But, she wrote, “since we could not understand a word of it we failed to see its interest.” Now they saw its interest: it was a way of doing philosophy that reconnected it with normal, lived experience.

They were more than ready for this new beginning. At school and university, Sartre, Beauvoir and Aron had all been through the austere French philosophy syllabus, dominated by questions of knowledge and endless reinterpretation of the works of Immanuel Kant.

Epistemological questions opened out of one another like the rounds of a turning kaleidoscope, always returning to the same point: I think I know something, but how can I know that I know what I know? It was demanding, yet futile, and all three students — despite excelling in their exams — had felt dissatisfied, Sartre most of all. He hinted after graduation that he was now incubating some new “destructive philosophy,” but he was vague about what form it would take, for the simple reason that he had little idea himself. He had barely developed it beyond a general spirit of rebellion. Now it looked as though someone else had got there before him. If Sartre blanched at Aron’s news about phenomenology, it was probably as much from pique as from excitement.

Either way, he never forgot the moment, and commented in an interview over 40 years later, “I can tell you that knocked me out.” Here, at last, was a real philosophy. According to Beauvoir, he rushed to the nearest bookshop and said, in effect, “Give me everything you have on phenomenology, now!”

What they produced was a slim volume written by Husserl’s student Emmanuel Levinas, La théorie de l’intuition dans la phénoménologie de Husserl, or The Theory of Intuition in Husserl’s Phenomenology. Books still came with their leaves uncut. Sartre tore the edges of Levinas’s book open without waiting to use a paper knife, and began reading as he walked down the street. He could have been Keats, encountering Chapman’s translation of Homer:

Then felt I like some watcher of the skies,

When a new planet swims into his ken;

Or like stout Cortez when with eagle eyes

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He star’d at the Pacific — and all his men

Look’d at each other with a wild surmise —

Silent, upon a peak in Darien.

Sartre did not have eagle eyes and was never good at being silent, but he was certainly full of surmises. Aron, seeing his enthusiasm, suggested that he travel to Berlin in the coming autumn to study at the French Institute there, just as he had done. Sartre could study the German language, read the phenomenologists’ works in the original, and absorb their philosophical energy from near at hand.

With the Nazis just coming to power, 1933 was not the perfect year to move to Germany. But it was a good time for Sartre to change the direction of his life. He was bored with teaching, bored with what he had learned at university, and bored with not yet having developed into the author of genius he had been expecting to become since childhood.

To write what he wanted — novels, essays, everything — he knew he must first have Adventures. He had fantasized about labouring with dockers in Constantinople, meditating with monks on Mount Athos, skulking with pariahs in India and battling storms with fisherman off the coast of Newfoundland. For now, just not teaching schoolboys in Le Havre was adventure enough.

He made the arrangements, the summer passed, and he went to Berlin to study. When he returned at the end of his year, he brought back a new blend: the methods of German phenomenology, mixed with ideas from the earlier Danish philosopher Soren Kierkegaard and others, set off with the distinctively French seasoning of his own literary sensibility. He applied phenomenology to people’s lives in a more exciting, personal way than its inventors had ever thought to do, and thus made himself the founding father of a philosophy that became international in impact, but remained Parisian in flavour: modern existentialism.

Excerpted from At the Existentialist Café: Freedom, Being & Apricot Cocktails by Sarah Bakewell. Copyright © 2016 Sarah Bakewell. Published by Alfred A. Knopf Canada, a division of Penguin Random House Canada Ltd. Reproduced by arrangement with the Publisher. All rights reserved.

Flying Books and Knopf Canada are hosting a celebration of Sarah Bakewell and her book (with apricot cocktails) at the Gladstone Hotel’s Melody Bar on April 13 at 6 p.m. Admission is free. Featuring an onstage dialogue between Bakewell (speaking on behalf of Jean-Paul Sartre), journalist Emily H. Keeler (Simone de Beauvoir) and writer Russell Smith (Albert Camus). More information at flyingbooks.ca.