Abortion Is Difficult
There is one thing that the extreme pro- and anti-abortion people can agree upon: that the issue is intellectually trivial, the correct answer blindingly obvious. They just disagree about which position is blindingly obvious and which stupidly evil.
I disagree, though. I think the issue of abortion is difficult. In fact, if you think the issue is easy, then I would say you're irrational. Anyway, here are some of the reasons it's difficult.
There are two main questions: (i) Do fetuses have a right to life? (ii) If fetuses have rights, is it still okay to abort them?
Start with (i). Here are some of the complexities:
Very few people think either that all fetuses have a right to life from the moment of conception, or that no fetuses have a right to life even one second before birth. Almost everyone has some intermediate position. Which means (i) becomes, for most, a line-drawing question; most "pro-life" people are people who draw the line relatively early, while "pro-choice" people draw it relatively later. But there is a continuous development from the time of conception to birth; there is no sudden, observable qualitative change. Which makes it bizarre that some people would think you are stupid or evil if you don't draw the line in the same place they do. Some candidate dividing lines and why they are problematic:
The moment when the fetus first has a heartbeat is morally irrelevant.
The time when it becomes viable doesn't seem morally relevant either, particularly since that time is relative to the current state of medical technology.
When the fetus becomes conscious would mark a morally relevant change, but that is not observable by us.
Whether it has brain waves would be relevant to its consciousness, but only as a weak necessary condition (there could be brain waves without consciousness).
When the fetus acquires a soul would be relevant, but that is also unobservable.
Even if the fetus is conscious at some time, that needn't mean it has the rights of a person; after all, nonhuman animals are conscious, but few ascribe them the rights of a person.
Maybe potential personhood confers the same right to life as actual personhood. Some people assert this very confidently; others deny it very confidently. There's some vague plausibility to either view.
But note that if one appeals to potential personhood, one has to hold that a normal person is the same entity as the fetus out of which that person developed. (I didn't say "same person" because maybe the fetus isn't a person.) Otherwise, it would be false that the fetus has the potential to later be a person. But this is non-obvious and depends on issues in the theory of personal identity. Personal identity is an incredibly challenging, controversial, paradoxical area of philosophy, in which there is no view that is not subject to powerful objections.
Maybe to have a right to life, you have to have a conception of yourself as a subject of experience continuing through time (Michael Tooley's view). This view has some plausibility. So does the negation of this view.
Tooley argues for it based on the premise that to have a right to x, one must be able to have desires about x. Maybe that's true. Or maybe one only needs to be such that one will naturally develop desires about x in the future if no one interferes.
More generally, there are about a million theories people can come up with about what it takes to have a "right", all with about the same vague plausibility. There is no consensus, and there are no good arguments establishing one account as better than all the others. No one should be confident of any theory of the basis of rights.
Some say that the moment of conception is special, because it is at that point that there is first an entity with a genuine potential to become a person. But this is not true. An egg cell and a sperm cell, where the sperm cell is swimming toward the egg, also have the potential to become a person. We tend to overlook this only because our ordinary conceptual scheme does not recognize the "sperm plus egg combination" as an object; but this is merely a matter of where we arbitrarily draw conceptual boundaries. There is no qualitative difference between the sperm-plus-egg and the just-fertilized-egg.
In case you are tempted to say that before fertilization, it is indeterminate which sperm cell will get to the egg: imagine there is an egg cell in a petri dish (prepared for in vitro fertilization), with just a single sperm cell about to fertilize it. It is implausible that the sperm-egg-combination is then an entity with the full rights of a person.
About question (ii), is it permissible to abort fetuses if they have a right to life?:
Thomson says it would be okay to expel the fetus from your body, even if it has a right to life, because its right to life does not imply that you have a duty to support its life using your own body; you own your body, and you don't have to let anyone use it, even if they need it to survive. (See the famous violinist hypothetical.)
This is a very right-wing libertarian argument, appealing to self-ownership and a purely negative conception of rights (yay). I notice, however, that many left-wing people seem to find this argument convincing, yet they reject the same sort of libertarian, "negative rights only" logic in other areas (e.g., when it comes to whether I have to use my labor or resources to support the poor).
Now leave aside ideology, if you can. It's plausible that you have a moral obligation to make some personal sacrifice to save the life of a person in need, where you are the only one able to save that person. But also, there is some limit to how much you can reasonably be expected to sacrifice. Which means that there is another line-drawing problem, and people could reasonably differ about how much a person can be obliged to sacrifice.
If you think it's obvious that carrying a fetus to term is an unreasonable sacrifice, note that it depends on what the reasons are for not wanting to carry it to term. There could be very serious reasons, but there also could be very bad reasons. So presumably the Thomson argument does not support very strong pro-choice positions wherein one has a right to an abortion for any reason.
Many people think that parents have special obligations to their children, such that you would have to do much more to save your child than you would have to do to save a stranger. Does this mean that a mother has special obligations to her fetus? Or do these obligations only kick in at birth?
Thomson says we "surely" don't have any special obligations unless we've voluntarily assumed them, but this is far from obvious.
If you have a right to abort due to the Thomson style of argument, this wouldn't be a right to kill the fetus per se; it would be a right to remove the fetus from your body (so if it can survive outside the womb, you can't go on to kill it). Q: traditional abortions kill the fetus first, in order to remove it; they do not remove the fetus first, where its death is then a side effect. Some people think that there is an important difference between killing X as a means to Y and killing X as a foreseen side effect of Y. Does this matter to the morality of abortion?
A final moral question: Suppose we are highly uncertain whether abortion is seriously wrong. What should public policy then be? Two prima facie plausible arguments:
It should be prohibited, because in general, if an action has a pretty good chance of killing some innocent people, that action will be prohibited (and rightly so), unless there is some extremely strong reason in favor of it. E.g., I can't play Russian roulette with innocent (unwilling) people, since the risk is too high. Since the abortion issue is difficult, we should conclude that there is a pretty good chance that abortion is morally comparable to killing innocent people, so it should be prohibited.
It should be permitted, because in general, a person should only be legally punished when the state is pretty sure that person has done something wrong. This reflects the conviction that it is worse to punish people unjustly than to fail to punish people justly, which explains why we have a presumption of innocence in the criminal justice system. Since abortion is a difficult issue, we should conclude that it is never beyond a reasonable doubt that any person who has had (or performed) an abortion has done anything wrong. I.e., they are in a position morally comparable to that of a person who is accused of murder but there is only something like a 50% chance that they did it. Such a person would rightly be let free.
I don't have many conclusions from all this. But here is one conclusion: If the abortion issue seems very simple and obvious to you, then you're probably a dogmatic ideologue, and your ideology is stopping you from appreciating this very subtle, complex question. Abortion is a highly intellectually interesting issue, connected with all sorts of important -- and very difficult and controversial -- issues: Issues about personal identity, potentiality, the foundation of rights, the physical basis of consciousness, the doctrine of double effect, special obligations to family, negative vs. positive conceptions of rights, and the problem of moral uncertainty.