Rabi was born in 1898, into an observant Orthodox Jewish family, in Rymanów—what was then part of Austrian-ruled Galicia and is today part of Poland. At his circumcision, he received the name Yisrael Yitzchak. Soon thereafter, his family emigrated to the U.S., settling into a small apartment on Manhattan’s Lower East Side before moving to the Brownsville neighborhood of Brooklyn.

Rabi’s parents fully maintained and cherished their Orthodox Jewish observance, and the family spoke Yiddish exclusively.

As a young boy, though, Yisrael Yitzchak quickly picked up English, and his interest in science led him to the public library and to tinkering with electronics. He was no mere dabbler. His first scientific paper, on the design of a radio condenser, was published in Modern Electrics when he was still in elementary school.

And his scientific explorations were not limited to any one field. When he read about heliocentrism, like many a young person coming to doubt the wisdom of his parents’ life choices, “Izzy”—how he came to be known at school—proudly declared himself an atheist. “It’s all very simple.” he informed his parents. “Who needs God?”

They were disappointed, as might be expected, but tolerant. He was only a boy, after all. And when his bar mitzvah approached, they asked him to prepare a bar mitzvah drasha. This was usually a scholarly discourse on a topic sourced in the Talmud for the guests, who were all religious people like Rabi’s parents. Rabi eventually agreed. And delivered a speech—in Yiddish—about how an electric light works.

“I talked about the carbon filament,” he recalled in his later years, “and then there was something I thought was very clever: getting the [electrical] lead out from the filament.”

Despite his 13-year-old freethinking ways, Rabi, even as a youth, came to mature in his worldview. And to feel the sense of awe that underlies deeply religious minds.

To be sure, he told Rigden, “When you are Orthodox, you say prayers for the new moon … [but] when you have an astronomical explanation, the rising of the moon becomes a sort of non-event.”

And yet, he recounted how, once he “was walking along and looked down the street, which was facing east. The moon was just rising. And it scared the hell out of me! Absolutely scared the hell out of me.” His inexplicable reaction to the grandeur of a moonrise was at odds with cold rationalism.

Many decades after his boyhood, he recalled how intrigued he had been as a youth, even an ostensibly atheist one, by the biblical creation account. “The first verses … were very moving to me as a kid. The whole idea of the Creation—the mystery and the philosophy of it. It sank in on me, and it’s something I still feel.”

“My early upbringing, so struck by God, the maker of the world, this has stayed with me,” he mused. “The idea of God,” he added, “helps you to have a greater feeling for the mystery of modern physics.”

Rabi expressed disdain for physicists who pursue their occupation for fun. “I knew other ways to have fun,” he said, drolly. “Physics has a much deeper emotional quality for me than that.”

Rabi considered physics as something that “transcended religion,” but didn’t replace it. Physics, he explained, “filled me with awe, put me in touch with a sense of original causes. Physics brought me closer to God.”

He recounted that whenever one of his students would come to him with a scientific project, he asked “only one question: ‘Will it bring you nearer to God?’” And, he added, “They always understood what I meant.”

After the war, Rabi returned to Columbia University, where he had taught in the 1930s. He became an outspoken critic of the continued development of nuclear weapons, joining with Oppenheimer and Enrico Fermi to oppose the development of the hydrogen bomb, 1,000 times more destructive than the bombs that had fallen on Hiroshima and Nagasaki.

A plan he had put forth to enlist all nations in forswearing development of such a weapon fell flat at the United Nations when the Soviet Union refused to be part of it.

In his final days, Rabi had the bittersweet experience of undergoing an MRI examination, the medical fruit of his prewar research. He was approaching death’s door—through which he passed on Jan. 11, 1988—but could take heart at what he had bequeathed to help save, not take, lives. “I saw myself in that machine,” he exulted. “I never thought my work would come to this.”

Rabi never returned to the Orthodox practice of his family and his youth. And yet, at the same time, he remained somehow conjoined with it. “Nothing in the world can move me as deeply as some of these Orthodox Jewish practices,” he confided.

“People go to Israel, to Williamsburg in Brooklyn, or to those places where Orthodox Jews go … and they pray and shake back and forth. Some people are appalled by it, but to me it is great. These are my people. I could join them, shake back and forth, and feel all right about it.”

He paused then to reassure his biographer that “I’m a scientist, which I firmly believe transcends—doesn’t oppose, but transcends—these particular things. I am of this … there is no question, but I’m not in it, couldn’t be in it. I love it and I respect it, but as a scientist I am at a more universal level … and this comes back to God.”

“Of this but not in it.” An apt capture of the subtle complexity of Rabi’s feelings toward his religious roots. What he realized was the truism that a scientific mindset needn’t preclude a spiritual one, indeed can be based on one. Human lives are lived in color, not black and white.

And so, while J. Robert Oppenheimer may have epitomized the “pure” scientist, his friend Rabi was, psychologically speaking, to borrow a phrase, a man in full.

In a revealing, pithy attestation, Rabi told his biographer: “There’s no question that basically, somewhere way down, I’m an Orthodox Jew […] In fact, to this very day, if you ask for my religion, I say ‘Orthodox Hebrew’—in the sense that the church I’m not attending is that one. If I were to go to church, that’s the one I would go to. That’s the one I failed. It doesn’t mean I’m something else.”