Imagine, wrote Thomson, that you awoke to find yourself lashed to a famous violinist. The violinist suffers from a lethal kidney disease, and because only your blood type can save his life, his admirers have kidnapped you and looped your circulatory systems together. If you consent to remain thus entangled for nine months, he will make a full recovery. Disentangle yourself, however, and he dies.

Thomson suggested that a woman facing an unintended pregnancy is in a similar position. Her body is effectively being held hostage, and while carrying the unborn life to term might be a heroic act, it cannot be required of her, any more than you could be required to meekly accept your fate as a prisoner of the violinist.

Provocative as it is, there are obvious problems with this analogy. It implies that there’s no difference between declining to provide medical treatment and taking a life directly, and no difference between the moral obligations owed a stranger and the obligations owed one’s own child.

The biggest difficulty, though, is that most women considering an abortion were not kidnapped and impregnated against their will. They freely chose the act that brought the fetus into being, and analogizing their situation to a kidnap victim implies a peculiar, almost infantilizing attitude toward female moral agency.

There is, however, one case where Thomson’s famous thought experiment has a real and gripping power: pregnancies that result from rape. Then the woman’s body has in a sense been kidnapped by her assailant, and the life inside her is the consequence of a violation rather than a choice.

From a rigorous anti-abortion perspective, that life has the same inalienable rights as any other innocent. But even the most rigorous abortion foe recognizes the unique agony — and perhaps, the political impossibility — involved in asking a woman to bear her rapist’s child.