The success of the film “12 Years a Slave,” which is up for the best picture Oscar at Sunday’s Academy Awards, has made an unexpected best-seller of the book on which it is based. Solomon Northup’s memoir, published in 1853, reads a bit like the best science fiction, in the sense that the world it depicts is fully recognizable, but with an odd twist that gives it a constant sense of being off-kilter.

Of course, the twist is slavery, so we are dealing with neither science nor fiction.

A recurring presence in the book is the law, which is almost a separate character, a kind of unholy ghost. Matter-of-factly, Northup shows how the legal system both constrained and institutionalized slavery, through a series of everyday prohibitions and permissions. (Warning: Spoilers ahead.)

Northup was originally persuaded to leave New York, his home, by two men who assisted in the kidnapping that forced him into slavery. To win Northup’s trust, the men made it clear that “it would be well, before leaving New York, to procure free papers.” Complying with the legal niceties, “they made an oath to certain facts showing that I was a free man.”

After Northup was whipped and stripped of those papers, he was initially sold to William Ford, whom he describes as “a model master.” But Ford ran into financial difficulty, and he sold Northup to John Tibeats, a man of great cruelty. Tibeats was unable to pay Ford the full amount and therefore owed him $400; that debt saved Northup’s life.

In one of the book’s most harrowing passages, Tibeats marched Northup to a tree to be hanged, placing his neck in a noose. Mr. Chapin, the overseer charged with protecting Ford’s legal interest, brought pistols to the scene. He told Tibeats that if he hanged Northup, Ford would lose his debt: “Until that is canceled you have no right to take his life,” Chapin said. “You have no right to take it any way. There is a law for the slave as well as for the white man.”

Northup was aware that if Tibeats ended up killing a slave, he might get away with it. “Had he stabbed me to the heart in the presence of one hundred slaves, not one of them, by the laws of Louisiana, could have given evidence against him.”

Northup points to the absence of legal protection to explain why so few slaves flee. Those who find a fleeing slave “have the right, either by law, or by general consent, to inflict discretionary chastisement upon a black man caught beyond the boundaries of his master’s estate without a pass, and even to shoot him, if he attempts to escape.”

Northup also has a lot to say about the uneasy relationship between justice and law. Responding to the widespread view that slaves are pretty happy, he contends that “they do not fail … to realize the injustice of the law which place it in (the slaveholder’s) power not only to appropriate the profits of their industry, but to subject them to unmerited and unprovoked punishment, without remedy, or the right to resist, or remonstrate.”

And he quotes one Mr. Bass, a white critic of slavery, complaining to a slaveholder, as saying that the law gives you the right to hold a black man, “but begging the law’s pardon, it lies.”

Northup’s rescue became possible only because he wrote a letter, mailed by Bass and explaining his plight, that came to the attention of Henry Northup, a member of the white family that had emancipated Northup’s father. Henry was able to find “among the statutes of the State an act providing for the recovery of free citizens from slavery.” Following the legal formalities, he succeeded in securing Solomon’s freedom.

One of the most important provisions of the Constitution, ratified after the Civil War as part of the 14th Amendment, forbids states from denying any person “the equal protection of the laws.” The equal protection clause is now understood as a general prohibition on discrimination, but its text is more specific. Somewhat mysteriously, it focuses on equality of “protection.” Why?

With the help of Northup’s memoir (and other contemporaneous sources), we can answer that question. Slaves lacked equal protection of the laws in the particular sense that horrible things could be done to them with impunity. Even in the aftermath of abolition, the newly freed slaves were highly vulnerable. The purpose of the equal protection clause was to reduce that vulnerability.

This conclusion has contemporary implications. All over the world, human beings remain vulnerable to private and public violence. In countless places, women, gays and lesbians, disabled people, and members of unpopular religious and ethnic groups are subject to “unmerited and unprovoked punishment, without remedy, or the right to resist, or remonstrate.”

No one should deny that institutionalized slavery was (and remains) unique. But wherever the law’s protection is selective and incomplete, we can find its horrific legacy.

By Cass R. Sunstein

Cass R. Sunstein, the Robert Walmsley university professor at Harvard Law School, is a Bloomberg View columnist. He is a former administrator of the White House Office of Information and Regulatory Affairs, the co-author of “Nudge” and author of “Conspiracy Theories and Other Dangerous Ideas,” forthcoming in March. ― Ed.