Discovery is rarely a one-person enterprise. Newton and Leibniz formulated calculus more or less simultaneously. The telescope was invented, like, five separate times in various countries. And in 1872, two different German mathematicians independently figured out how to define real numbers. Those two men, Georg Cantor and Richard Dedekind, struck up a fond friendship, but when Cantor reached out for Dedekind’s help with a nascent theorem, it set in motion one of math’s great controversies—one that finally seems to have been definitively settled.
Whenever Cantor met like-minded mathematicians, he was known to court them eagerly. He would show up at a collaborator’s residence at daybreak, excited to discuss some new idea he’d had, sometimes waiting for hours until they woke up. So it was with Dedekind. After their 1872 encounter in Gersau, Cantor took every opportunity to ask the older mathematician for advice.
In November 1873, Cantor began an exchange that would forever alter the course of human knowledge. “Allow me to put a question to you,” he wrote to Dedekind in a hastily penned letter. “It has a certain theoretical interest for me, but I cannot answer it myself; perhaps you can.”
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