The politics of abortion have become so heated that Tuesday's news about Missouri's intent to force the last abortion clinic in the state to shut down by the end of this week was only the second most important story of the day to touch on reproductive rights. First prize would have to go to the remarkable 20-page concurrence penned by Supreme Court Justice Clarence Thomas in an otherwise fairly low-stakes abortion case.

The concurrence raised sweeping moral and legal objections to abortion — but it went far beyond that, taking aim squarely at birth control, the purchase of which has been constitutionally protected since 1965, when Griswold v. Connecticut established a right to privacy in matters of marriage and sex. With this concurrence, Thomas may well be attempting to set the legal and constitutional agenda for the next generation of social conservatives. That the arguments he deploys in doing so are extraordinarily weak should ease no one's mind at all.

The case that served as the occasion for Thomas' moral musings was Box v. Planned Parenthood. The Box petitioners asked for review on two issues. The first involved the question of whether Indiana law may require health-care facilities to dispose of fetal remains in the same manner as other human remains (through burial or cremation). By a vote of 7-2, the Court ruled that it can. (For pro-lifers, this could potentially serve as one tiny step among hundreds in the direction of establishing fetal "personhood," which anti-abortion activists hope one day to use to get the procedure banned nationwide. But that day is still a ways off.)

The second question raised by the Box petitioners dealt with whether a state may prohibit abortions motivated solely by the race, sex, or disability of the fetus. On this matter the Court denied review, claiming that the Courts of Appeals must weigh in before the high court takes a position. But that didn't stop Justice Thomas from writing a concurrence in which he made quite clear that he thinks such restrictions on abortion are not merely permitted by law but quite possibly demanded by morality itself.

The concurrence sketches the history of eugenics in the United States, highlighting its ties to the movement for rights to birth control, especially in the writings of Margaret Sanger, the feminist icon and founder of Planned Parenthood. Sanger favored birth control as a means to eugenic ends, according to Thomas, including the elimination of undesirable traits from the human race through selective breeding, sterilization, and other forms of control over who gets to reproduce.

As an account of what Sanger and many other American progressives favored in the early decades of the 20th century, Thomas' essay is mostly unobjectionable. Eugenic ideas were common at the time, and often intertwined with toxic forms of racism and xenophobia, as well as hostility to and fear of the disabled. In Sanger's case, birth control was appealing, in part, because it would enable society more efficiently and effectively to rid itself of those who were racially, ethnically, or genetically "unfit."

When Thomas leaps forward to the present and attempts to draw related conclusions about the contemporary practice of abortion, he makes a worthwhile, if limited, point. There is indeed something eugenic-like about the way prenatal testing looks for anomalies in the developing fetus, with medical practitioners often subtly (or not-so-subtly) nudging expectant parents in the direction of terminating a pregnancy at the first indication of bad news. That should trouble us because of what it says about our willingness to distinguish between worthy and unworthy forms of human life, and about our eagerness to free ourselves (as individuals and as a society) of the burden of caring for those who require extra effort.

Yet Sanger's views and our own are far more different than Thomas lets on. For one thing, parents who choose to abort a pregnancy after learning that the fetus suffers from a serious genetic anomaly are not doing so in order to improve the genetic stock of humanity. They are doing so in order to be spared the crushing heartache and cost of raising a severely disabled child who may also end up condemned to a life of acute suffering. However morally complicated and questionable that choice might be, it has little to do with Sanger's explicitly eugenic aims.

Then there is the crucial distinction between, in the words of political theorist Jacob T. Levy, "the dispersed choices of individual persons," which is what leads to these decisions today, and "the aspirations for collective biosocial engineering by 20th-century authoritarians and progressives" — aspirations that often involved extraordinary acts of state coercion, including forced sterilization of the handicapped and members of disfavored racial groups.

Add it all up and Thomas appears to be quite wrong to suppose that allowing individuals to opt for abortions based on considerations of "sex, race, or disability … would constitutionalize the views of the 20th-century eugenics movement."

Things get even worse when Thomas attempts to apply these claims to contraception. If birth control still sometimes implied forced sterilization, Thomas would have a point. But birth control today is used exclusively to describe efforts by free men and women to control when a pregnancy will follow from sexual intercourse. The term is hardly ever used to describe an effort to "purify the race" by controlling who is permitted to reproduce. To insinuate otherwise, as Thomas does, is flatly outrageous — and most likely the product of a desire to elide the distinction between, and equate the moral dilemmas at play in, birth control and abortion.

That's a common move among certain elements of the religious right, especially Catholic conservatives, but it is nearly always unpersuasive. Abortion is morally fraught because it involves the taking of a life that may well possess the dignity that is the foundation of the right to life enjoyed by all human beings. Birth control involves the prevention of such a life from coming into being in the first place. It is thus an act with no negative moral consequence at all, at least when it comes to rights, because no one is harmed and no one's freedom is transgressed when one or both members of a sexually active couple decide to do what they can to avoid a pregnancy.

Only those who affirm a teleological view of human procreation will think that contraception transgresses morality, believing that a couple who uses it fails to fulfill their proper (and usually God-given) ends. But liberal democracies do not typically endorse the use of state power to back up the teleological moral convictions of one sect among many residing in the polity.

The prospect of disregarding and trampling on that distinction, like so many others, appears not at all to bother Clarence Thomas. And that fact should bother the rest of us perhaps more than any other.

CORRECTION: An earlier version of this article inaccurately described Margaret Sanger as advocating abortion. She was in fact a critic of the procedure. We regret the error.