The idea behind the Potentiality Principle (PP) is that potential personhood confers a right to life. For present purposes we may define a person as anything that is sentient, rational, and self-aware. Actual persons have a right to life, a right not to be killed. Presumably we all accept the following Rights Principle:
RP: All persons have a right to life.
What PP does is simply extend the right to life to potential persons. Thus,
PP. All potential persons have a right to life.
PP allows us to mount a very powerful argument, the Potentiality Argument (PA), against the moral acceptability of abortion. Given PP, and the fact that human fetuses are potential persons, it follows that they have a right to life. From the right to life follows the right not to be killed, except perhaps in some extreme circumstances.
But what is the argument for PP?
Consider post-natal humans from birth on to the age of seven or so. Do they have rights? Liberals, who believe in all sorts of rights, will presumably maintain that children have rights to health care, education, food, shelter, clothing and what all else. But underlying all of these is surely the right to life without which none of the others has any purchase. Now what is the basis for ascribing rights to neonates or one year olds? They cannot walk or talk, syllogize or philosophize. Chess is out of the question as is cell phone use. They cannot even control their bowels. They are helpless and in a sense good-for-nothing. They don't cook or clean or shop or work or pay taxes. They are more useless than cats who at least can fend for thmselves and control the rodent population. There is little that is actual about them. They don't realize much value or exercise the capacities that confer value or a right to life. And yet, as matter of fact, we consider them precious.
On what basis, then, do we ascribe value and rights to them? We value them for their promise and potentiality. We value them for what they will, in the normal course of events, become. I am not saying that there is nothing actual in young children that we value. We may value their openness to experience, their curiosity, their ability to learn, their sheer existence. But their potentiality outweighs their actuality and this potentiality is the ground of our ascription to them of the right to life and other rights that presuppose this fundamental right. So my argument goes like this:
1. We ascribe the right to life to neonates and young children on the basis of their potentialities.
2. There is no morally relevant difference between neonates and young children and fetuses.
3. Principles -- in this case PP -- should be applied consistently to all like cases.
4. We should ascribe the right to life to fetuses on the basis of their potentialities.
What I am arguing is that we already do accept PP and that we ought to be consistent in its application. To refuse to apply PP to the pre-natal cases is to fail to apply the principle consistently.