WE CALLED HIM the Professor. And he called my son Root, because, he said, the flat top of his head reminded him of the square root sign.

“There’s a fine brain in there,” the Professor said, mussing my son’s hair. Root, who wore a cap to avoid being teased by his friends, gave a wary shrug. “With this one little sign we can come to know an infinite range of numbers, even those we can’t see.” He traced the symbol in the thick layer of dust on his desk.

Of all the countless things my son and I learned from the Professor, the meaning of the square root was among the most important. No doubt he would have been bothered by my use of the word countless—too sloppy, for he believed that the very origins of the universe could be explained in the exact language of numbers—but I don’t know how else to put it. He taught us about enormous prime numbers with more than a hundred thousand places, and the largest number of all, which was used in mathematical proofs and was in the Guinness Book of Records, and about the idea of something beyond infinity. As interesting as all this was, it could never match the experience of simply spending time with the Professor. I remember when he taught us about the spell cast by placing numbers under this square root sign. It was a rainy evening in early April. My son’s schoolbag lay abandoned on the rug. The light in the Professor’s study was dim. Outside the window, the blossoms on the apricot tree were heavy with rain.

The Professor never really seemed to care whether we figured out the right answer to a problem. He preferred our wild, desperate guesses to silence, and he was even more delighted when those guesses led to new problems that took us beyond the original one. He had a special feeling for what he called the “correct miscalculation,” for he believed that mistakes were often as revealing as the right answers. This gave us confidence even when our best efforts came to nothing.

“Then what happens if you take the square root of negative one?” he asked.

“So you’d need to get – 1 by multiplying a number by itself?” Root asked. He had just learned fractions at school, and it had taken a half-hour lecture from the Professor to convince him that numbers less than zero even existed, so this was quite a leap. We tried picturing the square root of negative one in our heads: . The square root of 100 is 10; the square root of 16 is 4; the square root of 1 is 1. So the square root of – 1 is …

He didn’t press us. On the contrary, he fondly studied our expressions as we mulled over the problem.

“There is no such number,” I said at last, sounding rather tentative.

“Yes, there is,” he said, pointing at his chest. “It’s in here. It’s the most discreet sort of number, so it never comes out where it can be seen. But it’s here.” We fell silent for a moment, trying to picture the square root of minus one in some distant, unknown place. The only sound was the rain falling outside the window. My son ran his hand over his head, as if to confirm the shape of the square root symbol.

But the Professor didn’t always insist on being the teacher. He had enormous respect for matters about which he had no knowledge, and he was as humble in such cases as the square root of negative one itself. Whenever he needed my help, he would interrupt me in the most polite way. Even the simplest request—that I help him set the timer on the toaster, for example—always began with “I’m terribly sorry to bother you, but …” Once I’d set the dial, he would sit peering in as the toast browned. He was as fascinated by the toast as he was by the mathematical proofs we did together, as if the truth of the toaster were no different from that of the Pythagorean theorem.