Is Abortion Immoral?
Given the facts as we can at present prove them to be, there is no good reason to regard abortion as immoral. To demonstrate this we must first explain what it means for something to be moral or immoral, and also what abortion is.
The foundation of every moral system is the combination of values with facts. To disagree on a point of morality can thus be a dispute about the facts (such as the circumstances and consequences of an action), or a dispute about values, or both. If a disagreement hinges on facts, one case or the other is vindicated when both sides honestly investigate and acknowledge the facts that can be known. If a disagreement hinges on values, inquiry must be made as to why each holds the values they do. Values are either the conclusions derived from certain facts in combination with certain other values, or else they are fundamental. Disputes over values that are the conclusions derived from certain facts in combination with certain other values are resolved as for all other moral disagreements--by investigating the facts more carefully.
Disputes over fundamental values, however, are irreconcilable.
Both sides must agree to disagree, or develop a mutually agreeable compromise. It is even possible, within certain limits, to respect the individual moral sentiments of others even when we do not adopt those principles ourselves. But disputes that appear irreconcilable are not necessarily about fundamental values. They may simply result from either side failing to understand the reasons for either position, or from the inability to establish certain facts as true or false. Such cases must be resolved by first tolerating eachother or working out a compromise, while continuing to logically analyze the dispute and to investigate the facts. In law, this principle is manifest as moratoria, temporary injunctions, and holding suspects in custody (with or without allowance of bail), all of which being necessarily temporary solutions pending investigation, trial or judicial consideration. These observations must be kept in mind throughout this debate.
Those who argue that abortion is wrong generally base their argument on respect for individual human existence. Usually, this value, or something similar, is rightly assumed to be universally shared, and then the dispute arises only on matters of fact. I will continue this assumption. There are those, we can imagine, who not only have no value for respecting individual human existence, but also could not even in principle be persuaded to adopt such a value (e.g. by appealing to some other values they did possess which would be fulfilled by adopting a value for human life). But such people would not be persuadable on any point of morality anyway, rendering this debate of no use or interest to such a creature. We are thus speaking to, and for, everyone else.
Given the above, the required set of circumstances for abortion to be immoral are any which violate respect for individual human existence. The question here is thus not whether the value for individual human existence is justified (this will be assumed for this debate), but whether any circumstances of abortion contradict the object of that value. This is therefore a dispute about the facts, not values. Moreover, there are in almost all moral systems cases when killing is not immoral, and some when it is even moral. Self defense (or the defense of others) is the most prominent and relevant example here. But I imagine there will be no secular dispute in actual matters of self defense. In other words, I will assume for now that everyone agrees that abortion neither is immoral, nor should be illegal, when necessary to save the mother's life. This leaves only one issue for debate: whether elective abortion is immoral.