In Defence of Abortion

1) 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.[4]

Thomson takes it that you may now permissibly unplug yourself from the violinist even though this will cause his death: the right to life, Thomson says, does not entail 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."[5]

For the same reason, Thomson says, abortion does not violate the fetus's right to life but merely deprives the fetus of something—the use of the pregnant woman's body—to which it has no right. Thus, it is not that by terminating her pregnancy a woman violates her moral obligations, but rather that a woman who carries the fetus to term is a 'Good Samaritan' who goes beyond her obligations.

2) Third-party Participation - the "Expanding Child"

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.[7]

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