A general formulation of the Conservative Argument for the immorality of abortion argues that abortion is immoral because killing a fetus is the same as killing a person, which is clearly unacceptable, and any rights one has to one's own body is trumped by the fetus' right to live. Most arguments for or opposing abortion concentrate on the moral status of the fetus: is the fetus a person or not? However, Judith Jarvis Thomson argues, in the article "A Defense of Abortion," that even if the fetus is granted the same status and rights as any other established person, abortion is still morally permissible. In the article, Thomson provides many arguments from analogy, all intended to show that different variations of the Conservative Argument are unfounded.
Thomson begins by presenting her take on the 'Extreme View' for the immorality of abortion:
P1. A fetus is a human being from the moment of conception.
P2. Every human is a person with a right to life.
C1. Therefore, a fetus has a right to life from the moment it is conceived.
P3. Every person has a right to decide what happens in and to one's own body.
P4. However, a right to life always overrides a right to decide what happens in one's own body.
C. Therefore, abortion is always morally impermissible, even in cases where the woman will die if she does not abort the fetus.
Next, Thomson provides her rebuttal 'Expanding Child' scenario: Suppose you find yourself trapped in a tiny house with a growing child. This house is extremely small and the child is rapidly engorging. Already, you are pressed up against the walls of the house and in just a few minutes the child will grow enough that it will crush you to death. The child will not, however, be crushed to death, it will simply bursts from the house once it has grown large enough. It may be the case that a bystander cannot choose between saving your life or the child's life. However, Thomson argues, even though the child may be entirely innocent, it does not mean that you cannot intervene and kill the child to save your own life.
The expanding child scenario is an example of the argumentative technique known as an argument by analogy. This technique attempts to justify the moral assessment of a particular case by comparing it with a second scenario that virtually everyone will agree about. So, if the first practice is morally unacceptable and a second practice is morally analogous to the first, it can be concluded that the second practice is also morally wrong. Thus, the expanding child case can be seen as analogous to a case where a pregnant woman will die unless she aborts the fetus, which would survive if the woman were the one to die. Thomson takes the woman to be analogous to the house, both of which have a readily growing child within. She takes it that in the case of the expanding child, most will agree that the woman has the right to kill the expanding child to save her own life even though the child was innocent. She assumes that people will generally agree with the case because one would be acting in self-defense and only killing out of necessity to preserve one's life. Thus, in the case of a pregnant woman, the woman should also be allowed, if she chooses to do so, to save her own life by aborting the fetus.
The expanding child analogy is not Thomson's central argument for the permissibility of abortion in most cases; Thomson simply uses this scenario to refute the Extreme View. The remainder of Thomson's argument progresses through and eventually refuting various formulations of the Conservative Argument until she is able to reason that abortion is morally permissible in most cases. The expanding child case is one early step, or sub-argument, along the way to her final conclusion. After portraying the ineffectiveness of the Extreme View she then progresses to condemn the Weakened View, which says that it is permissible to save the mother's life in the expanding child case or that of pregnant mother threatened by a growing fetus, but the abortion may only be performed by the mother herself. So even though the expanding child case is not Thomson's main argument it, nonetheless, plays an important role in the argument by establishing that a woman being crushed by a rapidly expanding child has the right to terminate the child's life and the implication of this is that a woman whose life is in danger is morally permitted to terminate the fetus that is growing inside of her.
I find Thomson's self-defense argument to be both effective and persuasive. I have always sided with a woman's right to choose what is best for her, but I understand why many people get very emotional about abortion - to them it is equivalent to killing a one year old baby, eight year old child, or thirty year old adult. That being said, what I can't comprehend is why some people don't think a woman has a right to an abortion in self-defense. It is uncontroversial that someone has the right to stop a home intruder even with deadly force to save their own or their family's lives, which is similar to Jarvis' Expanding Child scenario. Both seem to be equivalent to having a right to choose an abortion to save one's own life, yet all of a sudden there is a disconnect in moral acceptability. The difference in the intruder case is that it is the intruder is at fault while the expanding child and a fetus are innocent. I think that is where the strength of Jarvis' argument lies, because many people will agree that it is okay to kill the innocent expanding child in order to save one's own life. But, perhaps fervent anti-abortion people would not agree that it is acceptable to save one's life in this case either?
Judith Jarvis Thomson, "A Defense of Abortion" in What's Wrong: Applied Ethicists and Their Critics, ed. David Boonin and Graham Oddie (New York: Oxford University Press, 2010), 89-98.