Some pro-life objections hinge on the idea that there is a morally relevant difference between the potential of a gamete (sperm or ovum) and the potential of a zygote, even if there are no more neurological or psychological features in the zygote than in the gamete. This is sometimes referred to as “modal potentiality” or “kind potentiality,” which some pro-life philosophers believe is enough to endow personhood on the fetus. I restate this principle as the following:
“The zygote/embryo/fetus (ZEF) is a person with a right to life in a way a gamete is not, because the ZEF is an individual human entity of the kind that can potentially have rationality, thoughts, and other valuable mental features.”
In other words, the ZEF may not have any morally relevant mental features now, but it is “of the kind” of being that, given enough time and progression, would have those mental features. This is not true of the gamete “left alone.” You might say the ZEF is a “human organism” but the gamete is merely biologically human.
This argument presupposes that being “of the kind” of being that possesses the qualities we agree make human lives uniquely important (mental states like desires, relationships, thoughts, etc.) grants that being the moral status it would have if it does have those qualities. But anyone can immediately challenge this presupposition, because it’s obvious we don’t assign a certain status or rights to an entity based on whether the entity has the “modal potential” for a certain morally relevant quality, but rather whether or not it actually has that quality.
It’s easy to understand this point with some simple examples. For instance, we don’t desire to eat a piece of cake because the piece is “of the kind of food that at its best is delicious.” We desire to eat the slice because it is delicious; further, if the cake were not delicious, we wouldn’t want to eat it, regardless of whether cake as a “kind” is a delicious food. What matters is whether the individual piece of cake possesses the quality of deliciousness.
Similarly, consider the right to consent to sex. Children are “of the kind” of being (persons) that can at some stage in mental development consent to sex. But we don’t view that as important at all in deciding whether children ought to have some right to consent to sex. What matters is that children don’t currently have the mental capacity to make consent meaningful, not whether they are of the kind of being that would at some point have it.
So, to make the objection more obvious, critics of abortion reason this way:
“ZEFs (but not individual sperm or egg cells) are of the kind of being that has morally relevant mental states.. And if something is “of the kind” of being that grows to have morally relevant mental states, that grants that being the moral status it would have if it did have those states. And so ZEFs should be treated as if they are rational or have the rights of rational beings.“
My cake and consent examples show that this type of reasoning is faulty:
“A piece of cake is the kind of thing that’s delicious. Cakes are a kind of food that’s delicious. Therefore, this piece of cake is delicious.“
“ZEFs are the kind of being that can consent to sexual relations (this is true if they are also the kind of beings that are rational, as advocates of the above argument claim, and they do). If a being is the kind of being that can consent to sexual relations, then that being has the right to consent to sex. Therefore, ZEFs can consent to sex.“
With regards to abortion, the ZEF does not have any mental life, desires or dispositions; this is not true of newborns, children, or adults. The fact that the ZEF could or will develop into a being that has those mental features does not mean that the ZEF in the body of a woman who wants to get an abortion should therefore be viewed as morally equivalent to a a being tha does have those mental features. Quite the opposite, in fact: The fact the ZEF does not have those features is grounds to consider it morally different from a newborn or child, regardless of what “kind” of being it is.