John Urschel has heard it a lot: “You’re that football player who’s really smart.” It’s an accurate description — Urschel, a former offensive lineman for the Baltimore Ravens, is a doctoral candidate in mathematics at MIT — but that description makes him wince.

Instead, he offers this: “I’m looking forward to being considered that mathematician who used play football.”

In July of 2017, Urschel retired from the NFL after three seasons. He hasn’t played football since. Not even a casual game in the park? Nope. “This isn’t something my friends and I do for fun,” he says.

Besides, Urschel has been busy with something else — his memoir, “Mind and Matter.” Written with his wife, journalist Louisa Thomas, the book chronicles his life in both sports and academics and explains how and why, in the end, he chose math over football. Yes, it had something to do with concussions, but that’s not all of it.

Lounging at his in-laws’ house in the District (his father-in-law is journalist Evan Thomas), the 27-year-old Urschel is both soft-spoken and emphatic. At 6-foot-3 and well over 200 pounds, he takes up a lot of the couch, but not in the daunting way you might expect of someone whose job it once was to daunt. Bespectacled, Urschel sports a trim beard that’s more professorial than James Harden’s. There’s no millennial irony to it — or to Urschel. Sure he plays that generational favorite Settlers of Catan, but this is a guy who mostly likes to play with, think about and discuss one thing: numbers.

When asked what he likes to do for fun, his immediate answer is, “math!” He carries a notepad with him just in case he’s struck by an idea. He wanted his first book to be a pure math book, and at signings he’s been known to inscribe the memoir he ended up writing with “fun integrals” and “important constants” that he’s excited to explain to anyone who wants to geek out with him.

Urschel grew up in Buffalo. His parents, who separated when Urschel was 3, encouraged his two-track learning from an early age. His mother, a nurse-turned-lawyer, urged him to pursue his interests in math with games and puzzles, and later advanced course work. “If I am a true outlier,” he writes in his book, “it is because of her — an African-American single mother who loved math but was discouraged from it, who wanted me never to feel that any door was closed to me.” His father, a thoracic surgeon and former linebacker at the University of Alberta, pushed him academically, too. But he was also concerned about his son’s conditioning, and his time with young Urschel involved visits to the gym as well as to the library.

Urschel’s mother was eager for her son to go to the Ivy League, but Urschel had his eye on a football powerhouse: Penn State. There he met Joe Paterno. “I know who you are,” Urschel recalls the legendary coach telling him at their first meeting, when Urschel was still in high school. The encounter affected him deeply, Urschel says, conjuring up a memory of what JoePa told him at that meeting: “You know what, Urschel, you’re a tough football player and a high character guy. You’re going to be a leader of this Penn State football team. You’re going to be a captain.”

At the time, he says, Urschel thought Paterno was off the mark. After all, he says now, he was just “a 2-star recruit from Buffalo.” “But, what do you know? It turned out he was right after all.”

It’s a rare moment of immodesty from Urschel, who stayed at Penn State through the team’s sexual abuse scandal despite opportunities at Stanford and elsewhere. (He never met former assistant coach Jerry Sandusky, recalling him only as the man a teammate referred to as “the old guy who’s in the weight room sometimes.”) Urschel says he has both sympathy for the victims and loyalty to Penn State. Even now, he says, he sees his years as a Nittany Lion as some of the best of his life.

The Ravens picked Urschel in the fifth round of the 2014 draft. Though thrilled to be chosen — for one, he wanted to make enough money to help his mom — Urschel wasn’t ready to give up math, and didn’t. While his teammates were playing video games, watching TV and whatever else, Urschel was writing and publishing papers in math journals. In 2016, he enrolled in MIT. But there was always a nagging worry about injury.

“Being an offensive linesman is like being in dozens of small car crashes, only you’re not the driver, you’re the car,” Urschel writes in his book. In August 2015, the car was nearly totaled: After a serious concussion, Urschel failed a baseline test of his cognitive function three times in a row. Even when back on the field, he felt unusually anxious and aggressive.

More concerning, he was unable to do high-level math. His skills returned eventually, but he was shaken by the experience. Less than two years later, he finally said, enough. On July 27, 2017, he announced his retirement.

It was the right decision, Urschel says, but it also raised some profound personal questions. As he writes in his book: “How could I best serve the talents I was born with? How could I best serve my family and society? What was I willing to risk?” He also thought a lot about “the opportunity I had to reach kids who might otherwise turn away from math,” he writes, “especially African-American kids, who had too few examples of black mathematicians.”

In December of 2017, his daughter, Joanna, was born. He and Louisa, whom he’d met while she was writing a profile of him, decided it was time to make a change. He enrolled full-time in MIT, with a doctorate in applied mathematics expected in 2020.

Now when Urschel speaks, there’s a lightness about him. He is literally lighter — he says he’s lost 35 pounds in recent months — but there’s also that lightness that comes with the lifting of a burden. Urschel no longer has to decide between football and math, between numbers in a playbook and equations on a blackboard.

“In many ways I hope this is my swan song,” he says, tapping a copy of his book. It marks the end of that fascinating duality, and the beginning of a more singular purpose: “to focus on mathematics and quietly try to do good things to inspire young people with respect to math.”

Nora Krug is an editor and writer at Book World.