(Bloomberg Opinion) -- It’s increasingly clear that Roe v. Wade, the U.S. Supreme Court decision protecting a woman’s right to choose abortion, is in jeopardy. But what is Roe all about? Privacy? Liberty? Women’s equality?

Its survival may depend partly on the answer, so let’s go back to first principles.

The Roe opinion, written by Justice Harry Blackmun in 1973, was entirely about privacy. As Blackmun put it, the right of privacy “is broad enough to encompass a woman’s decision whether or not to terminate her pregnancy.”

For two reasons, that’s awkward. First, the U.S. Constitution does not protect a general right of privacy at all. Second, any right of privacy, if it does exist, would not seem to encompass the right to choose abortion. Privacy usually refers to the right to control access to personal information.(1) What does abortion have to do with that?

In 1992, the court switched gears. In an opinion jointly written by Justices Sandra Day O’Connor, Anthony Kennedy and David Souter, the authors spoke not of privacy but of liberty. They said that “there is a realm of personal liberty which the government may not enter.”

They added that a “mother who carries a child to full term is subject to anxieties, to physical constraints, to pain that only she can bear” — suffering that “is too intimate and personal” for government to override.

The word “liberty” is in the Constitution, so it’s an improvement on “privacy.” In ordinary language, liberty does include the right to make intimate and personal decisions.

Still, the Constitution does not protect all such decisions. You can’t marry your mother, or more than one person. There is no constitutional right to smoke cigarettes. Liberty can be curtailed when the interests or rights of others are at stake. What about the fetus?

In a short, cautious essay published in 1985, eight years before she joined the Supreme Court, Ruth Bader Ginsburg struggled with these issues. She made two points. First, she argued that Roe badly overreached. In her words, “the court ventured too far in the change it ordered.”

Second, she thought that the court should have stressed women’s equality. In her words, it “presented an incomplete justification for its action.”

In Ginsburg’s view, the justices ventured too far because the Texas law at issue in the Roe case was so extreme, forbidding abortion except to save the life of the mother. The court could have invalidated that law more narrowly — for example, by saying that women must be allowed freedom of choice in cases of rape and incest. (On this count, her argument has contemporary relevance in view of the similar scope of recently enacted prohibitions on abortion.)

It could have done that without establishing a broad right to choose abortion. By doing so much so soon, Ginsburg wrote, the court “stimulated the mobilization of a right-to-life movement and an attendant reaction in state legislatures.”

Drawing on the work of law professor Kenneth Karst of the University of California at Los Angeles, Ginsburg added a more substantive point: the court should have spoken in terms of “discrimination against women.”

True, only women can become pregnant. But in her account, it is not enough for states “to charge it all to women’s anatomy.” It is the law, not anatomy, that prohibits abortion. In thinking about the right to choose, she would have “added a distinct sex discrimination theme.”

Ginsburg noted that doing so would have required a renovation of discrimination law. Because men can’t get pregnant, restrictions on abortion do not treat women differently from men — and so are not seen to pose a discrimination issue at all. (That was true in 1985, and it is true now.)

In Ginsburg’s view, that is a big mistake. Current law might be thought to have it backwards: The very reason that restrictions on abortion are a form of sex discrimination is that only women can get pregnant. (A tempting question: If men could get pregnant, how many states would prohibit abortion? It’s unanswerable, of course.)

Ginsburg did not deny that if restrictions on abortion are seen as discriminatory, they might still be upheld. The government always gets a chance to argue that it has a sufficiently strong justification for its actions, even if they seem discriminatory.

No one should disparage the sincere moral commitments of those who believe that even at an early stage, fetuses count as human beings, and that the law should protect them. Insisting on the relevance of sex equality is not enough to demonstrate that those who hold those commitments are wrong.

But as early as 1985, Ginsburg emphasized the centrality of “a woman’s autonomous charge of her full life’s course” — of “her ability to stand in relation to man, society, and the state as an independent, self-sustaining, equal citizen.” In 2019, no discussion of abortion should neglect that topic, even if it is occurring in courts of law.

(1) In speaking of privacy in Roe, the court was following its reasoning in its 1965 decision in Griswold v. Connecticut, where it spoke of the constitutional right to privacy in ruling in favor of a right to use contraception. The use of privacy has often been criticized there as well,and rightly so.

To contact the author of this story: Cass R. Sunstein at csunstein1@bloomberg.net

To contact the editor responsible for this story: Jonathan Landman at jlandman4@bloomberg.net

This column does not necessarily reflect the opinion of the editorial board or Bloomberg LP and its owners.

Cass R. Sunstein is a Bloomberg Opinion columnist. He is the author of “The Cost-Benefit Revolution” and a co-author of “Nudge: Improving Decisions About Health, Wealth and Happiness.”

©2019 Bloomberg L.P.