Anyone who hopes that the abortion debate will not degenerate into a mere shouting match must welcome Judith Jarvis Thomson's article. It injects a powerful new argument into the debate. Thomson's case against restrictive regulation of abortion rests on three ideas she finds very plausible. They are: "First, restrictive regulation severely constrains women's liberty. Second, severe constraints on liberty may not be imposed in the name of considerations that the constrained are not unreasonable in rejecting. And third, the many women who reject the claim that the fetus has a right to life from the moment of conception are not unreasonable in doing so." It follows that restrictive regulation of abortion may not be imposed in the name of the claim that the fetus has a right to life from the moment of conception.

A recent papal encyclical, Evangelium Vitae, argues that restrictive regulation of abortion should be imposed in the name of that claim. According to the encyclical, the doctrine that the fetus has a right to life from the moment of conception is "knowable by reason itself." Thomson takes the claim that this doctrine is knowable by reason to imply the claim that it is unreasonable to believe that the doctrine is false. Read this way, the encyclical is committed to the view that the many women who reject the claim that the fetus has a right to life from the moment of conception are unreasonable in doing so, which contradicts Thomson's third idea. But surely Thomson's third idea, or something close to it, is correct. Perhaps some people who reject the claim that the fetus has a right to life from the moment of conception are unreasonable in doing so. However, the idea that all women who reject the claim are unreasonable in doing so has no plausibility at all. Thus interpreted, the encyclical simply fails to acknowledge the reasonable pluralism of moral doctrines that exists in democratic societies in which free inquiry into moral issues is permitted to flourish.

Thomson’s political morality strikes me as problematic. 

So we should accept Thomson's third idea. And her first idea seems to me undeniable. But her second idea's principle of political morality strikes me as problematic. Consider the Mormons who once practiced polygamy. It is clear that laws prohibiting polygamy severely constrained their liberty. It also seems to me very plausible to suppose that many of them could have reasonably rejected the considerations typically invoked to support such laws. It is not unreasonable to deny that monogamy is the best form of marriage or the only morally permissible form of marriage. Yet I think there is nothing morally wrong in enacting laws prohibiting polygamy, even if such laws cannot be justified to all who are constrained by them in terms of considerations they cannot reasonably reject. So I am persuaded that Thomson's principle is false.

The example shows that this principle has not always been honored in the practice of American politics. If I am right, it also shows that severe constraints on liberty may sometimes be imposed in the name of considerations that the constrained are not unreasonable in rejecting. Whether such constraints may be imposed in the case of abortion will have to be determined by other features of that case. Knowing of no further features of the case that decisively rule out imposing such constraints, I conclude, tentatively, that those who press in the political arena for restrictive regulation of abortion in the name of the doctrine that the fetus has a right to life from the moment of conception would, if they succeeded in getting such measures enacted, violate no correct principle of political morality and so do no wrong. But I myself hope that such efforts will not succeed because I favor women having a duly qualified legal right to decide whether or not to abort at least during the first trimester of pregnancy.