The logic of the pro-life position is expressed below in five slightly different (very simple) arguments, all of which are logically valid. If the premises are true, then so are the pro-life conclusions.
Argument One
1. The unborn (i.e., human embryo or fetus) is a human being.
-- This premise is confirmed by the scientific facts of human embryology, which show that the unborn from conception is a distinct, living and whole human organism -- a member of the species Homo sapiens at the earliest stages of development.
2. It is prima facie wrong to kill a human being.-- This is difficult to deny. To those who say it is only prima facie wrong to kill some human beings (excepting the unborn and perhaps others), the SLED test shows that discrimination between groups of human beings is not justified. Since it is wrong to kill already-born human beings, and since no morally relevant difference exists between already-born (e.g., a toddler) and unborn (e.g., a 7-month-old fetus) human beings, then is is also wrong to kill unborn human beings. So the premise is true.
3. Therefore, it is prima facie wrong to kill the unborn.
-- Our conclusion tells us that as a general rule (prima facie) it is wrong to kill the unborn. Killing human beings may sometimes be justified (e.g., just war, capital punishment, self-defense), but the unborn is innocent and abortion is almost always intentional killing, so it is not clear how this particular example of killing human beings -- elective abortion -- could possibly be justified. Some abortion defenders, following Judith Jarvis Thomson, might suggest a sophisticated argument from bodily autonomy (an approach I address here).
Argument Two
1. The unborn is a human being.
2. All human beings have a right to life.
3. Therefore, the unborn has a right to life.
-- The killing of abortion clearly seems to violate the unborn human being's right to life.
This second argument is virtually identical to the first argument, since one could say that for a being to have a right to life is for it to be prima facie wrong to kill that being.
Argument Three
1. Elective abortion is the intentional killing of an innocent human being.
-- Facts about abortion methods show that abortion is (almost always) the intentional killing of the unborn (the extremely-rare hysterotomy procedure might be an exception, and there are failed "abortions" that do not actually succeed in killing the unborn).
-- Science shows that the unborn is a human being.
2. It is morally wrong to intentionally kill an innocent human being.
-- Again, it is obviously wrong to intentionally kill an innocent adult human being, so if there are no morally relevant differences between classes of innocent humans (e.g., between newborns and the unborn) that would justify intentional killing, then this premise is true. All possible contenders for being a "morally significant difference" seem arbitrary, self-serving, and have unacceptable implications -- they are degreed properties, thereby undermining the equality of persons, and they inevitably exclude obvious examples of rights-bearing persons, such as infants and the temporarily comatose. So we may conclude that there are no morally significant differences, and that human beings have moral value simply by virtue of being human. Thus, since it is wrong to kill an adult, it is wrong to kill a fetus.
3. Therefore, elective abortion is morally wrong.
-- This argument gets us to a more precise conclusion than the first two: we don't have to do any more work post-conclusion to arrive at "abortion is wrong." But notice that more work has to be done in the premises.
Argument Four
Philosopher Patrick Lee formulates the pro-life argument as follows:
1. "Intentionally killing an innocent person always is morally wrong."
-- Lee cites consequentialist moral theories (e.g., utilitarianism) as the main challenger to this premise. On such a view, for example, it might be right to lynch an innocent man in order to appease an irrational mob, given the overall consequences weighed against the consequences of not lynching him. Few people find this theory persuasive.
2. "Abortion is the intentional killing of an innocent person."
-- To defend this premise, one must show that what abortion kills (the unborn) is a human being (a scientific fact) and that all human beings are "persons," i.e., bearers of fundamental dignity and a right to life (a moral claim). Lee notes that some abortion defenders may deny this premise by denying that abortion is intentional killing; this is the bodily autonomy argument offered by Thomson.
3. "Therefore, abortion is always morally wrong."
Argument Five
Philosopher Francis Beckwith offers this argument:
1. "The unborn entity, from the moment of conception, is a full-fledged member of the human community."
-- By "full-fledged member of the human community" Beckwith means a rights-bearing person like you and me. Thus, to defend this premise, one needs to use both the facts of science (to show that the unborn is a human organism) and moral reasoning (to show that all human beings have the moral status of a "person").
2. "It is prima facie morally wrong to kill any member of that community."
-- This seems to be entailed by what we understand "a full-fledged member of the human community" to be -- someone whom it is prima facie wrong to kill.
3. "Every successful abortion kills an unborn entity, a full-fledged member of the human community."
4. "Therefore, every successful abortion is prima facie morally wrong."
However the basic pro-life argument is formulated, it uses the facts of science combined with sound moral reasoning to reach an inescapable conclusion about the correct treatment of unborn human beings. Defenders of abortion who aspire to think about this issue rationally (a necessity if one cares about the truth) have their work cut out for them.
Update: See two more pro-life arguments -- dealing explicitly with personal identity -- here.
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