His was Nicolas Bourbaki. Perhaps you were a young mathematician in the middle of the 20th century and noticed his name as a student in textbooks, read his work described in research papers, or even caught professors mentioning him with reverence. He was strict, meticulous, and visionary when it came to mathematics like no other person. However, if you searched for his picture, there was nothing to be found. If you went searching for his biography, you'd find contradictions. And if you ever wrote him a letter, well—no one would answer. Because Nicolas Bourbaki was a fictional phantom, a genial phantom, to be sure, created by a group of French mathematicians who concluded that the best way to revolutionize mathematics was not as individuals but collectively. In the 1930s, disillusioned with the sloppy, fragmented way math was being presented, a group of clever young intellectuals—André Weil, Henri Cartan, and Jean Dieudonné among them—came together to retackle the rules. Instead of their own names, they credited the work to "Nicolas Bourbaki," an entirely fictitious mathematician. And the world swallowed it. They wrote in Bourbaki's name, producing Éléments de mathématique, a colossal multi-volume work that reshaped mathematics with lean, axiomatic exactness. Their ideas underpinned the modern style of thinking in algebra, topology, and analysis. They invented new terminology, clarified established notions, and, to a great extent, built the mathematical vocabulary we use today. Bourbaki remained an enigma for decades. Even as rumors spread, the group played along with the joke—sending fake wedding invitations, writing fictitious obituaries, and insisting (with a straight face) that he was very much real. Eventually, the world caught on. The secret was revealed, and Bourbaki was exposed as the greatest hoax in mathematical history. But by then, his influence was undeniable. So, did Nicolas Bourbaki exist? Not in the way you’d expect. But his influence? Very real.