According to Judith Thomson, one can reasonably assert that the fetus has a right to life, and one can also reasonably deny it. But the possibility of denial trumps the possibility of assertion. Those who assert the fetus's right may not severely constrain the liberty of a pregnant woman by forbidding abortion so long as she can reasonably deny the ground for the constraint.

There is no epistemological escape from our moral conundrums.

Thomson argues this way because she wants to establish the woman's right to abortion without relying on the controversial claim that the fetus has no right to life. But her strategy is problematic. The question whether the woman can reasonably deny the fetus's right is itself controversial, despite Thomson's peremptory way with it.

The Catholic claims that the fetus's right to life, like other fundamental moral propositions, is revealed to us by reason. (Many non-Catholics also believe this, or something similar.) Thomson retorts that "There is nothing unreasonable or irrational in believing that the doctrine [of the fetus's right] is false." This retort proves either too much (if "unreasonable" means the same as "irrational") or too little (if "unreasonable" adds something). There is nothing irrational in denying that adults have a right to life. This threatens to make the law against murder problematic. If the law against murder is justified because it is unreasonable to deny that adults have a right to life, then the question arises whether it may not be similarly unreasonable to deny the fetus's right.

Answering the question whether one can reasonably deny the fetus's right requires a discussion of what is "reasonable" moral belief. This raises in turn the deepest issues about how we come to know any moral proposition at all. In the end, the question whether one can reasonably deny the fetus's right to life is likely to be just as difficult as the question whether the fetus actually has the right. (On some views, it will turn out to be the very same question.) Thomson has made no progress.

Perhaps, despite her skirmishing, Thomson isn't really concerned after all with the question of reasonableness of belief in the sense that raises these deep issues. Perhaps her real claim is just that we cannot severely constrain liberty on the basis of a proposition which many thoughtful, decent people deny. But why not? Thoughtful, decent people can be mistaken, and they can even be unreasonable or morally blind on some topics. Also, while many thoughtful, decent people deny the fetus's right to life, many other thoughtful, decent people affirm it. If those who affirm the fetus's right are in the majority, why should they not forbid abortion?

Note that this is not just a case of the majority limiting a woman's right in order to promote their own preference. The majority are acting on a reasonable belief (according to Thomson) that they are protecting other rights. I do not see how we can forbid the majority to prevent what they believe are massive violations of the right to life, unless we are prepared to claim they are mistaken.

Thomson holds up to scorn Bossuet's remark that "I have the right to persecute you because I am right and you are wrong." But Bossuet's central point is correct. There is no epistemological escape from our moral conundrums.