Quite often, in the abortion debate, one hears the “potential Mother Theresa” or potential human (being) argument.
Potentiality is quite problematic. After all, any given human is also a potential mass murderer or potential Hitler. Potentiality, as a concept, is rather dubious at best.
The idea is that killing a foetus is killing a potential human person or being. Let’s unpack this a little.
The first thing to say about this, as is commonly stated, is that it is then wrong, by extension, to use contraception. This can be further taken as being an argument against abstinence:
The strongest objection to the argument from potential is that it seems to make contraception, and even abstinence, prima facie morally wrong. If the objection to abortion is that it deprives the zygote of a “future like ours,” why, it maybe asked, cannot the same complaint be made of contraceptive techniques that ill sperm, or prevent fertilization? Why don’t gametes have “a future like ours”?… so if abortion is seriously wrong because it kills a potential person, then the use of conceptive is equally seriously wrong. In using spermicide, one commits mass murder! 
L.W. Sumner makes a similar accusation:
The above essay uses the following as a way of defining potential, which is worth laying out:
Rather, potential, in the way I am using it here and the way I assume most advocates of the argument from potential use the concept, refers to, as Stephen Buckle puts it, a certain being’s “potency… the power it [actually] possesses in virtue of its specific constitution”  to grow into a being of a certain sort. That is, X is a potential Y if X possesses the power to become Y; that X will become Y, if it lives long enough. In this way, a caterpillar is a potential butterfly (since it possesses the power to become a butterfly; it will become a butterfly if it lives long enough), as a child is a potential adult. A fetus is a potential person in this way; a fetus may not just possibly become a person, it will become a person, if its growth is unfettered and if it lives long enough.
Which sounds pretty fine until you realise that the most sense t make of this comes from determinism. if followed properly, it means that a zygote’s potential will depend entirely on its exact DNA and environment, and so a given zygote most certainly wouldn’t potentially be a “Mother Theresa”-type person unless it contained a very specific set of DNA and was to be subject to very particular environmental variables.
In very simple terms, though, I treat a (chicken’s) egg very differently to how I treat a chicken.
Take a human. Take Trump. He is now President. In previous years, he was a potential President, though no one would ever have accorded him such a title! Would he have commanded the same legal treatment as a mere reality TV mogul as he has now, as POTUS? Should he have been given the same rights? As a child, should he have been able to command the US forces?
You could argue that the POTUS does not deserve different treatment by society and law anyway, but that’s another thing. The point is, we would not have said at any point during Trump’s former life, “Oh you can’t do that…” or “We can’t treat him like that because he has the potential to be the POTUS.”
Indeed, as David Boonin says:
It certainly is not true of properties in general that if a given individual potentially has a given property, then the individual already has this property. Prunes have properties that plums do not have, wine has properties that grape juice does not have, and adults have properties that fetuses do not have. Perhaps a proponent of the potentiality argument could complain that these examples appeal exclusively to nonmoral properties. But the claim is equally unacceptable if it is limited to moral properties: Adults have moral responsibilities that children do not have, and older people cultivate moral virtues they did not have when they were younger. The question, then, is whether, given all this, there is any reason to think the assumption true of rights in general, or of the right to life in particular.
It only appears that people use the potential argument in the context of abortion, and that should tell you something. When the application of a particular mechanism is confined to a single instance, then it would appear that that instance is something of a shoehorning.
But what I really wanted to say in this post is pretty straightforward: If we were to say that we shouldn’t terminate a “potential” human life, wouldn’t that make me guilty by omission of not having sex with as many women as possible? Should that be the new policy of pro-life evangelicals? “Shag for life!!”
I wonder if pro-life evangelicals have a higher rate for sexual offences, or a libido that goes through the roof? If they really feel strongly about the potential of human life, they should be shagging all over the shop.