It is a grainy video clip, shot with a shaky camera. A reporter offscreen asks ominously, “Is there anything else that’s going to come out about you that we don’t already know?” The woman laughs nervously, “You know, I don’t think so, but who knows?” It’s clear that she was kidding, but in the context of this particular political advertisement, the line comes off sounding , well, conspiratorial.

The woman is Elizabeth Warren, and the brief dialogue comes from an attack ad launched by her rival for a senate seat in Massachusetts, Scott Brown. The ad, entitled “Who Knows?”, attacks Warren personally for claiming to be Native American — “something genealogists have zero evidence of.” The ad refers to a months-old political controversy, in which Warren got in trouble when opponents claimed that she had misrepresented herself as Native American to advance her career.

Oddly enough, “Who Knows?” called to my mind a book I read this summer. The book, “Passing Strange” by Princeton professor Martha Sandweiss, is about the first director of the United States Geological Survey, a noted explorer and scientist named Clarence King 1862. It is impossible to read anything about King, who was also an art critic and staple of high society, without coming across what Secretary of State John Hay said about him — he was “the best and brightest man of his generation.”

Clarence King had fair skin and blue eyes. I only mention this because — and here’s where the story gets interesting — for 13 years, King “passed” for black. He was married to a black woman, Ada Copeland, who was born a slave in Georgia, and he lived with her and their five children in Queens. To Ada, he was James Todd, a black Pullman porter (which explained his long absences from home, during which he was off being important and publicly white). Only on his deathbed in 1901 did King reveal his secret to Ada — that he was wealthy, educated, and, most importantly, white.

“Passing Strange” tells a fascinating love story, which manages to tie in race, poverty, and politics. At times, it is a little slow, but it is flawlessly researched and quite well-written.

I bought “Passing Strange” after glancing at the back cover. A prominent white government official who convinces his wife, children and neighbors that he is not only of a different background, but of a different race? How could it be done? Face paint, a mask? The answer, it turns out, is both less dramatic and more common than I had known. To become black in the late 1800s, all King had to do was claim to be black. It is important to remember that during this time, anyone with a drop of black blood — a single great-grandparent, say — was technically classified as black. For the multitudes who dwelled in the racially ambiguous middle ground of mixed-race forebears, they could “pass” for whichever race they wanted. The world is not simply black-and-white, yet when it was defined as such, anyone with neutrally colored skin was left in a predicament. As Baz Dreisinger wrote in The New York Times, “[R]ace is not really about skin color. If it were, the blond-haired, blue-eyed Walter White, for instance, could never have identified himself as ‘a Negro,’ served as executive secretary of the N.A.A.C.P. or written this paradoxical sentence: ‘The traits of my race are nowhere visible upon me.’ Race is the emperor’s new clothes: we don’t see it; we think it.”

In a culture where African-Americans were discriminated against by the color of their skin, who would voluntarily cross the color line? King did — because of love. “King loved Ada, and she loved him back,” Sandweiss wrote. Only an abiding love — and more than a touch of eccentricity — could cause King to live a double life and deceive every single person he knew.

Obviously, our conception of race is malleable. And this brings us back to Elizabeth Warren. It would be wrong, and frankly offensive, to claim that Warren spent decades “passing” as Native American. It was never a major part of her life, and indeed Harvard hired her with no knowledge of her ethnicity. Yet she has been vilified — and continues to be attacked — for supposedly attempting to pass.

As law professor Kevin Noble Maillard wrote in The New York Times, “For the Cherokee Nation, Warren is ‘Indian enough’; she has the same blood quantum as Cherokee Nation Chief Bill John Baker. For non-Natives, this may be surprising. They expect to see ‘high cheekbones,’ as Warren described her grandfather as having, or tan skin. They want to know of pow wows, dusty reservations, sweat lodges, peyote and cheap cigarettes. When outsiders look at these ostensibly white people as members of Native America, they don’t see minorities.”

Frankly, it doesn’t matter what they see. The labels “black,” “white,” and “Native American” are — and have been for centuries — subjective. The stories of Clarence King and Elizabeth Warren teach us that race is fundamentally undechipherable. Judging simply by the color of someone’s skin — or claiming that someone is not minority “enough”, or even attempting to tell someone else what race she is — is misguided.