Students of moral theology know it as "The Principle of the Practical Doubt." Everyone knows it as a dictum of common sense.
A good example of it is the hunter who does not know if what is moving behind the bush is a bear or a man. May he shoot before he finds out? Or does he have to clear up the doubt first? Similarly, if a building is about to be demolished, but there is reason to think some people may still be inside, may one go ahead and demolish the structure, or must the doubt be cleared up first?
The answers are obvious. This is called the Principle of Practical Doubt, because the doubt is not about the moral teaching involved (namely, that it is wrong to kill the innocent), but rather about the practical circumstances -- in other words, might a person be killed in this situation as a result of this action?
The 1973 Roe vs. Wade decision legalizing abortion violated this principle. The decision says,
"We need not resolve the difficult question of when life begins. When those trained in the respective disciplines of medicine, philosophy, and theology are unable to arrive at any consensus, the judiciary, at this point in the development of man's knowledge, is not in a position to speculate as to the answer." [410 U.S. 113, 159]
Notice that the Court did not say, "The unborn are not human." Instead, it said, "We don't know. We're not qualified to say." In other words, there was a practical doubt.
At the same time, the decision said, "The word "person," as used in the Fourteenth Amendment, does not include the unborn." [410 U.S. 113, 158]
In other words, you can destroy them anyway, despite the doubt.
In reality, of course, there is no doubt. If we did not know when and how human life begins, we could hardly conceive children in vitro, or have a court rule, as happened in New Jersey, that an abortion is "the legal execution of a human being" (Loce).
Yet for those who want to raise all kinds of questions about the humanity of the unborn child, the Principle of the Practical Doubt provides a powerful pro-life argument. In the encyclical The Gospel of Life, the Holy Father referred to this principle in the following words,
"From the standpoint of moral obligation, the mere probability that a human person is involved would suffice to justify an absolutely clear prohibition of any intervention aimed at killing a human embryo" (EV #60).
If, in other words, you kill what may be human, you declare by that action your readiness to kill what is human. Roe vs. Wade would have been a step closer to moral sanity had it said, "We know for sure that the unborn are not human; therefore, we are not obliged to protect them as persons." But it didn't. Instead, it embodied a willingness to kill those who are human, thereby putting all of us in danger.