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. Does the person-plant who now develops have a right to the use of your house? Surely not--despite the fact that you voluntarily opened your windows, you knowingly kept carpets and upholstered furniture, and you knew that screens were sometimes defective.The homeowner : person-plant :: woman : fetus analogy is supposed to show that a woman who has taken all reasonable precautions against getting pregnant, but does get pregnant because of faulty contraception, is not morally obligated to keep the fetus, just as the homeowner is not morally obligated to let the person-plant grow in her house.
Now, suppose that the person-plant growing in the homeowner's house is making the homeowner feel better. That is to say, suppose that the homeowner used to suffer from allergy symptoms, such as hives, hay fever, coughing, etc. But now that the person-plant is growing in her house, all the allergy symptoms are gone. The homeowner's health and quality of life have improved dramatically as a result of the person-plant's presence in her house.
Now does the homeowner have a moral obligation to let the person-plant grow in her house?
If you are inclined to answer yes, then consider the following: fetal cells in a woman's body can have either positive or negative effects on her health.
Now, suppose that we can tell in advance whether the cells of a fetus will have beneficial or harmful effects on a woman's health. If we accept Thomson's homeowner : person-plant :: woman : fetus analogy, are we committed to saying that it would be morally permissible to abort the fetuses that will have harmful effects on a woman's health and morally impermissible to abort the fetuses that will have beneficial effects on a woman's health?