The first step in thinking clearly about the issue is to clarify the meanings of the terms used, particularly “human life”. It is very easy to fall into the logical fallacy of equivocation when using the term “human life”, e.g.
It is clear that both the sperm and the egg are genetically human and that they are both alive in a biological sense. However this is not what we mean by “human life” in a moral sense. If it were then male masturbation would be genocide and female menstruation would be murder. It is also clear that the appendix is genetically human and alive, but nobody argues for a ban on appendectomies to protect the “human life” of the appendix.
There are two meanings of “human life” that are easily confused, the first meaning illustrated with the examples above refers to genetically human life and is a biological use rather than a moral one.
The second meaning of “human life” is membership of the class of beings to whom we owe moral obligations. This second meaning is the ethically and morally relevant one that we are referring to when we say it is wrong to take a “human life”.
We would presumably consider it equally wrong to kill an intelligent alien visitor without good cause, regardless of the non human nature of his DNA.
So what do we mean by “human life” in the ethical/moral sense
Philosophers debate at length the characteristics required to be members of the moral community and become entitled to the rights associated with it. e.g.
Mary Warren’s list of characteristics:
1. Consciousness (of objects and events external and/or internal to the being), and in particular the capacity to feel pain;
2. Reasoning (the developed capacity to solve new and relatively complex problems);
3. Self-motivated activity (activity which is relatively independent of genetic or direct external control).
4. The capacity to communicate, messages of with an indefinite number of possible contents on indefinitely many possible topics.
5. The presence of self-concepts and self-awareness.
However, there is an obvious criticism that the criteria listed are derived by observing the properties of a class of entities that are deemed to belong to the moral community and since this class excludes foetuses the argument carries no more weight than a simple assertion that foetuses are not members of the moral community.
Another problem with such criteria is that they not only conclude, inevitably, that a foetus has no right to life, they also entail the conclusion that a new born infant or a mentally disabled adult and many animals have no right to life either. A conclusion that offends our moral intuition.
Fortunately, we can avoid getting drawn into the complex philosophical debates about what constitutes “human life” in a moral sense by looking at the problem from the opposite end, where emotions do not cloud reason so much, that of human death.
If a fully functioning human being, with all the rights associated with that status, is involved in an accident they can be kept in a state of biological life via medical machinery almost indefinitely. However, it is generally accepted that if there is no cerebral brain wave activity then there is no “human life” in the moral sense. Turning off the life support machines is not murder but simply the deactivation of biological processes sustained by external means.
If this were not the case then it would be necessary to keep the corpses of everyone who “died” in hospital on life support machines forever or be guilty of murder!
Having established that human life,in the moral sense, requires cerebral brain wave activity the question becomes much simpler, when does the foetus develop the sort of cerebral brain wave activity that indicates human life?
This is a question of biology, rather and philosophy and it appears in the foetus between twenty four weeks and thirty weeks of conception.
(Pro-life claims of earlier activity appear to be scientifically invalid.)
A foetus younger than this is not a human life in the ethical sense and can have no more right to life than the body kept warm on a life support machine. The woman having an abortion is no more guilty of murder than the doctor who turns off the life support machine of a brain dead accident victim.
It is tempting if you currently hold anti-abortion views at this point to revert to the potential for human life argument and try to distinguish the case of the dead man on the life support machine. After all he has no potential for life, whereas the foetus has potential for a full human life.
However, we have already refuted the argument for rights for biological entities with the potential for human life on the grounds it would make menstruation murder and masturbation genocide. With advances in biological science even non embyonic cells now have the potential for human life. Can we seriously consider cutting nails or hair as murder!
To conclude, a foetus up to 24 weeks is not a “human life” in the moral sense, it is a genetically human biological entity with the potential for “human life”, but such an entity has no right to life.
Abortion of a foetus up to 24 weeks does not involve the violation of any human rights and libertarians should support the woman’s right of self ownership to abort a foetus if she wishes to.