In the 1980’s I would have said, and probably did say, that although abortion is a repulsive procedure, it is morally permissible, and therefore should be legally permissible. In the three decades since then, I’ve come to a very different conclusion. Here is my reasoning, in outline. I am happy that my reasoning has been guided by the Catholic Church, but it does not rest in any way on the Bible, revelation, or infallible authority — it rests, as any moral reasoning should, on pure reason.
I expect some people will find some holes in my argument. Please be so good as to point them out; this exposition will be very terse, and it is a work in progress.
1. Modern science does indeed show us that a new human life begins to be at conception: when a human egg and sperm unite, a new individual life form of the species Homo sapiens begins to exist, with its own unique DNA.
However (as, unfortunately, many pro-life people fail to recognize) we cannot leap from this scientific result to the moral conclusion that the newly conceived human has a right to life. Science is value-free. It tells us nothing about whether this human life, or any human life, has rights. That is a philosophical question.
2. Human beings have rights, including a right to life, by virtue of being persons: individual substances of a rational nature (Boethius’s definition of “person”). By “substance” I mean an existing being or thing. By “rational” I mean capable of discerning right from wrong, of choosing good and avoiding evil. By “nature” I mean the intrinsic or essential properties of a being — as the etymology of the word would suggest, what the being is born with — although the nature predates the time of birth.
If I ever should meet an “intelligent” being of another species, perhaps an alien from another world, this is the criterion I would think appropriate for deciding whether it has full moral rights, what we call “human rights”: is that being capable of morality? The capacity to feel pain endows a being with animal rights, but not human rights (or else let’s all become vegetarians). The capacity to calculate (as in some forms of artificial intelligence, either now or in the future) is simply not morally relevant. But the capacity to act as a moral agent seems to be necessary and sufficient. If an agent is treating me morally, I should treat it likewise. If it does not and cannot, it would seem unfair for me to be obligated to recognize its rights, when it does not and cannot recognize mine. So a rational nature — by which I mean a nature capable of supporting moral thought and action — is one that entitles the being possessing it to full human rights.
3. The life form that begins to be, when a human egg and sperm unite, is an individual substance of a rational nature, though not yet actually rational.
The unborn human has a rational nature, but that rationality is not yet realized: it is not yet actual.
A person sleeping also is not being actually rational, but that person has a right to life.
Fine, you may say, but before going to sleep, the person was actually rational, and that actual rationality confers a right to life which persists even during sleep.
But children, also, are not actually rational, and have not yet been actually rational; and still they have a right to life. Children have a rational nature: if they develop as they should, they become rational. They become morally capable and morally responsible at a time traditionally called the “age of reason,” roughly 12 to 14 years of age. But before that, they already have human rights. They have these rights, not because they are actually rational, or have ever actually been rational, but because of their rational nature.
4. Therefore, the unborn human being also has human rights, including a right to life.
5. The right of privacy does not trump the right of life, any more than I have a right to shoot people to prevent them from seeing me naked.