In "A Defense of Abortion", Thomson grants for the sake of argument that the fetus has a right to life, but defends the permissibility of abortion by appeal to a thought experiment:
You wake up in the morning and find yourself back to back in bed with an unconscious violinist. A famous unconscious violinist. He has been found to have a fatal kidney ailment, and the Society of Music Lovers has canvassed all the available medical records and found that you alone have the right blood type to help. They have therefore kidnapped you, and last night the violinist's circulatory system was plugged into yours, so that your kidneys can be used to extract poisons from his blood as well as your own. [If he is unplugged from you now, he will die; but] in nine months he will have recovered from his ailment, and can safely be unplugged from you.
Thomson says that you can now permissibly unplug yourself from the violinist even though this will cause his death: this is due to the right to life, which does not include the right to use another person's body, and so by unplugging the violinist you do not violate his right to life but merely deprive him of something—the use of your body—to which he has no right. "[I]f you do allow him to go on using your kidneys, this is a kindness on your part, and not something he can claim from you as his due."
For the same reason, Thomson says, abortion does not violate the fetus's legitimate rights, but merely deprives the fetus of something—the use of the pregnant woman's body and life-support functions—to which it has no right. Thus, by choosing to terminate her pregnancy, a woman does not violate any moral obligation; rather, a woman who carries her pregnancy to term is a 'Good Samaritan' who goes beyond her obligations.
Thomson criticizes the common method of deducing a woman’s right to abort from the permissibility of a third party committing the abortion. In almost all instances, a woman’s right to abortion may hinge on the doctor’s willingness to perform it. If the doctor refuses, then the woman is denied her right. To base the woman’s right on the accordance or refusal of a doctor, she says, is to ignore the mother’s full personhood, and subsequently, her rights to her body. Thomson presents the hypothetical example of the ‘expanding child’:
Suppose you find yourself trapped in a tiny house with a growing child. I mean a very tiny house, and a rapidly growing child—you are already up against the wall of the house and in a few minutes you’ll be crushed to death. The child on the other hand won’t be crushed to death; if nothing is done to stop him from growing he’ll be hurt, but in the end he’ll simply burst open the house and walk out a free man.
Thomson concedes that a third party indeed cannot make the choice to kill either the person being crushed or the child. However, this does not mean that the person being crushed cannot act in self-defense and attack the child to save his or her own life. To liken this to pregnancy, the mother can be thought to be the house, the fetus the growing-child. In such a case, the mother’s life is being threatened, and the fetus is the one who threatens it. Because for no reason should the mother’s life be threatened, and also for no reason is the fetus threatening it, both are innocent, and thus no third party can intervene. But, Thomson says, the person threatened can intervene, by which justification a mother can rightfully abort.
Continuing, Thomson returns to the ‘expanding child’ example and points out:
For what we have to keep in mind is that the mother and the unborn child are not like two tenants in a small house, which has, by unfortunate mistake, been rented to both: the mother owns the house. The fact that she does adds to the offensiveness of deducing that the mother can do nothing from the supposition that third parties can do nothing. But it does more than this: it casts a bright light on the supposition that third parties can do nothing.
If we say that no one may help the mother obtain an abortion, we fail to acknowledge the mother’s right over her body (or property). Thomson says that we are not personally obligated to help the mother but this does not rule out the possibility that someone else may act. As Thomson reminds, the house belongs to the mother; similarly, the body which holds a fetus also belongs to the mother.
To illustrate an example of pregnancy due to voluntary intercourse, Thomson presents the ‘people-seeds’ situation:
Again, suppose it were like this: people-seeds drift about in the air like pollen, and if you open your windows, one may drift in and take root in your carpets or upholstery. You don’t want children, so you fix up your windows with fine mesh screens, the very best you can buy. As can happen, however, and on very, very rare occasions does happen, one of the screens is defective; and a seed drifts in and takes root.
Here, the people-seeds flying through the window represent conception, despite the mesh screen, which functions as contraception. The woman does not want a people-seed to root itself in her house, and so she even takes the measure to protect herself with the best mesh screens. However, in the event that one finds its way in, unwelcome as it may be, does the simple fact that the woman knowingly risked such an occurrence when opening her window deny her the ability to rid her house of the intruder? Thomson notes that some may argue the affirmative to this question, claiming that “...after all you could have lived out your life with bare floors and furniture, or with sealed windows and doors”. But by this logic, she says, any woman could avoid pregnancy due to rape by simply having a hysterectomy – an extreme procedure simply to safeguard against such a possibility. Thomson concludes that although there may be times when the fetus does have a right to the mother's body, certainly in most cases the fetus does not have a right to the mother's body. This analogy raises the issue of whether all abortions are unjust killing.
Critics of Thomson's argument generally grant the permissibility of unplugging the violinist, but seek to block the inference that abortion is permissible by arguing that there are morally relevant differences between the violinist scenario and typical cases of abortion, one notable exception being that of Peter Singer who claims that, despite our intuitions, a utilitarian calculus would imply that one is morally obliged to stay connected to the violinist.
The most common objection is that Thomson's violinist argument can justify abortion only in cases of rape. In the violinist scenario, you were kidnapped: you did nothing to cause the violinist to be plugged in, just as a woman who is pregnant due to rape did nothing to cause her pregnancy. But in typical cases of abortion, the pregnant woman had intercourse voluntarily, and thus has either tacitly consented to allow the fetus to use her body (the tacit consent objection), or else has a duty to sustain the fetus because the woman herself caused the fetus to stand in need of her body (the responsibility objection). Other common objections turn on the claim that the fetus is the pregnant woman's child whereas the violinist is a stranger (the stranger versus offspring objection), or that abortion kills the fetus whereas unplugging the violinist merely lets him die (the killing versus letting die objection).
Defenders of Thomson's argument reply that the alleged disanalogies between the violinist scenario and typical cases of abortion do not matter, either because the factors that critics appeal to are not genuinely morally relevant, or because those factors are morally relevant but do not apply to abortion in the way that critics have claimed. Thomson's defenders also point to her 'people-seeds' argument as a strong analogy to typical cases of abortion.
Thomson's article, by positing a moral justification for abortion even if one grants a fetal right to life, opened up a new avenue in the philosophical debate about the ethics of abortion. Critics of her view have formulated many objections to her argument, and defenders have responded in kind in a back and forth that continues in philosophy journals even now. For more detail about how this debate has progressed beyond Thomson's article, see the article about the philosophical debate over abortion.