Harriet Tubman is a brilliant choice for the first woman to go on the face of a modern Federal Reserve note — and for more reasons than one. The former slave became a conductor on the Underground Railroad, a co-conspirator of John Brown, and a spy for the Union. Reviewing a biography of her issued a few years ago, our Carl Rollyson couldn’t help but wonder whether she was “the greatest spy this country has produced.” Harriet Tubman would be a fit figure for our currency even were she not a woman.

One of the things about the choice is that it will cause schools to put a focus on Tubman. It will illuminate the cruelty of slavery and the peculiar disabilities it brought to Tubman’s life; as a girl she was struck with a heavy weight thrown by a slave owner, precipitating sleep and other disorders that plagued her for the rest of her life. She was a true leader (John Brown called her “general”), one of the few escaped slaves who went back — repeatedly — to lead others to freedom. It is widely remarked that Tubman never lost a person consigned to her custody.

And yet there was a mystery to Tubman’s life that Mr. Rollyson, writing in the Sun, reckons none of Tubman’s biographers has ever solved. “Indeed,” he writes, “they do not seem to perceive this mystery at the heart of Tubman’s life. Why did she remain illiterate?” Was it that learning to read and write was forbidden to slaves? But what about after the war? Mr. Rollyson quotes one historian, Jean Humez, as suggesting that the “most likely” reason was that providing for her dependents left her without time or energy.

Mr. Rollyson finds that “unconvincing.” He notes that a woman as “determined” (and as brilliant) as Tubman could easily have learned to read and write. He suggests that she could have written her own autobiography (Frederick Douglass’s “Narrative of the Life of Frederick Douglass” and Harriet Jacobs’s “Incidents in the Life of a Slave Girl” are towering works). “Not learning to write,” Mr. Rollyson reckons, “meant that Tubman seemed to put herself at the mercy of mediating voices, no matter how much she could shape an interviewer’s questions or dictate her own version of events.”

Then Mr. Rollyson offers his own theory. He says that “the spy’s power and place in history depend in part on secretiveness — on floating stories and preserving, in today’s parlance, ‘deniability.’ Because she herself provided no written record, Tubman could avoid being caught in a contradiction or a lie. Discrepancies could always be attributed to the mediators.” The twilight nature of Tubman’s heroism was understood at the time, and remarked on in the incredible letter sent to her by Douglass himself (and quoted in full in this morning’s Wall Street Journal).

“The difference between us is very marked,” Douglass wrote to Tubman. “Most that I have done and suffered in the service of our cause has been in public, and I have received much encouragement at every step of the way. You on the other hand have labored in a private way. I have wrought in the day — you in the night. I have had the applause of the crowd and the satisfaction that comes of being approved by the multitude, while the most that you have done has been witnessed by a few trembling, scarred, and foot-sore bondmen and women, whom you have led out of the house of bondage . . .”

Which brings us to a final irony in respect of money itself, which is, after all, the medium on which our republic is going to give Tubman her glory. Tubman’s penchant for silence was a sign not of her weakness but of strength. It is also a feature of the truest of all money (and the greatest of all central bankers). Of the two classic forms of specie, it has been said, silver is speech, silence is golden. Let us hope that the accession of Harriet Tubman to the public eye will inspire not only the millions who use the money with her face on it but the central bankers who issue it.