As the title makes plain, Thomson argues that abortion is not impermissible. More precisely, she argues for the conclusion that abortion is sometimes permissible; she grants that there are scenarios in which obtaining an abortion would be immoral. What is especially novel is the manner in which Thomson constructs her argument. She begins the essay by pointing out that the debate over abortion seems to many people to hinge on whether or not the fetus is a person. Most feel that if we could only determine the answer to that puzzle, the implications for abortion would be clear; namely, that if fetuses are persons then abortions must be impermissible, and that if fetuses are not persons then abortions must be permissible. Thomson, though, thinks that reasoning in this way is misguided, or at very best is incomplete. In light of this, she begins by conceding the issue of personhood to her opponent; she assumes, for purposes of argumentation, that the fetus is a person from the moment of conception. She attempts to show that even if this concession is made, abortion is morally permissible in many cases.
Thomson first reconstructs the argument that opponents of abortion who rely on the personhood of the fetus must have in mind when they contend that the practice is always impermissible. A crucial premise in that argument is that the right to life must always outweigh any other rights with which it conflicts. Thomson believes that premise to be false, and deploys the first of several thought experiments of which she makes use throughout the paper to persuade her readers to that end. The experiment is designed to show that there are scenarios wherein it is permissible to take an action in order to vindicate one�s own rights (importantly, a right other than the right to life) even though doing so violates an innocent person�s right to life. Following this, Thomson moves on to a more detailed discussion of the right to life, wherein she attempts both to define the notion, and to point out that many other commonly held beliefs about the strength and implications of such a right must be false. It might be thought that having the right to life means that one has the right to whatever is necessary in order to sustain one�s life. Thomson thinks that this must be mistaken, as scenarios like the violinist case show. Alternatively, one might think that having the right to life means that one has the right not to be killed. Again, though, Thomson thinks that the violinist case shows this to be false; surely one can unplug oneself from the violinist, even though doing so kills him. If one attempts to alter the definition by suggesting instead that having the right to life means having the right not to be killed unjustly, then one has done little to advance the debate on abortion. After all, whether the killing of the fetus is just or unjust is precisely what is at issue.
Thomson further argues that even if women are partially causally responsible for the presence of the fetus, by voluntarily engaging in intercourse with the full knowledge that pregnancy might result, it does not thereby follow that they bear a special moral responsibility toward it. Here she makes use of the window thought experiments to demonstrate that partial causal responsibility for an outcome does not bear on one�s having moral obligations regarding the outcome. Recall, though, that Thomson does think that some abortions are immoral; not all reasons justify ending the fetus� life, and no reasons justify ending the fetus� life if it can be detached from the mother without killing it.