In Ted Honderich (ed.), The Oxford Companion to Philosophy, Oxford, 1995, pp. 2-3
Those who defend women's rights to abortion often refer to themselves as 'pro-choice' rather than as 'pro-abortion'. In this way they seek to bypass the issue of the moral status of the foetus, and instead make the right to abortion a question of individual liberty. But it cannot simply be assumed that a woman's right to have an abortion is a question of individual liberty, for it must first be established that the aborted foetus is not a being worthy of protection. If the foetus is worthy of protection, then laws against abortion do not create 'victimless crimes' as laws against homosexual relations between consenting adults do. So the question of the moral status of the foetus cannot be avoided.
The central argument against abortion may be put like this:
It is wrong to kill an innocent human being.
Defenders of abortion usually deny the second premiss of this argument. The dispute about abortion then becomes a dispute about whether a foetus is a human being, or, in other words, when a human life begins. Opponents of abortion challenge others to point to any stage in the gradual process of human development that marks a morally significant dividing-line. Unless there is such a line, they say, we must either upgrade the status of the earliest embryo to that of the child, or downgrade the status of the child to that of the foetus; and no one advocates the latter course.
The most commonly suggested dividing-lines between the fertilized egg and the child are birth and viability. Both are open to objection. A prematurely born infant may well be less developed in these respects than a foetus nearing the end of its normal term, and it seems peculiar to hold that we may not kill the premature infant, but may kill the more developed foetus. The point of viability varies according to the state of medical technology, and, again, it is odd to hold that a foetus has a right to life if the pregnant woman lives in London, but not if she lives in New Guinea.
Those who wish to deny the foetus a right to life may be on stronger ground if they challenge the first, rather than the second, premiss of the argument set out above. To describe a being as 'human' is to use a term that straddles two distinct notions: membership of the species Homo sapiens, and being a person, in the sense of a rational or self-conscious being. If 'human' is taken as equivalent to 'person', the second premiss of the argument, which asserts that the foetus is a human being, is clearly false; for one cannot plausibly argue that a foetus is either rational or self-conscious. If, on the other hand, 'human' is taken to mean no more than 'member of the species Homo sapiens', then it needs to be shown why mere membership of a given biological species should be a sufficient basis for a right to life. Rather, the defender of abortion may wish to argue, we should look at the foetus for what it is - the actual characteristics it possesses - and value its life accordingly.
Rosalind Hursthouse, Beginning Lives (Oxford, 1987).
Judith Jarvis Thomson, 'A Defense of Abortion', in Peter Singer (ed.), Applied Ethics (Oxford, 1986).
Michael Tooley, Abortion and Infanticide (Oxford, 1983).