When I say that a particular ability is in a thing's nature, I mean that just by virtue of what that thing is, it can perform certain functions. What is in a particular entity's nature is available simply by observation. For example, it is in a dog's nature to bark, to bury items, to chase rabbits, and do all manner of things. It is in a cat's nature to mew, to be independent, to catch mice, and to murder you in your sleep (okay, maybe not that last one). And human beings have certain capacities, too, such as the capacity to walk, to speak, to engage in higher-level thought, to be rational, to be conscious, to fall in love, etc. There are literally hundreds, if not thousands, of capacities. This is also not a Christian convention -- the concept of nature and capacities goes back at least to Aristotle, though Aristotelian thought has permeated Christianity quite a bit (in particular, read Aristotle's Metaphysics).
Talk of capacities is just another way to talk about what is in a thing's nature, though more specifically as there are different degrees that capacities come in, and different types of capacities that a thing can have. Capacities can also be thought of in terms of potency -- a capacity a thing has gives it the potential to develop or become something. Russell DiSilvestro, in his book Human Capacities and Moral Status (Springer Science + Business Media B.V. 2010, p.17), says:
A capacity is the metaphysical ground or truth-maker of a true conditional statement about a thing. For example, the capacities of sub-atomic particles are what make statements describing what would happen to these particles in different circumstances true. Both ordinary language and the language of philosophy and science are suffused with statements about capacities. For example, you have the capacity to speak English, my pet rat has the capacity to experience pain, and salt has the capacity to dissolve in water. Such capacity-statements typically have a structure containing three elements: one which refers to the capacity itself, another which refers to the bearer of the capacity, and a third element which refers to what the capacity is a capacity for.There is a bit of a difficulty in philosophy, in that different philosophers often use different terms to refer to the same thing. So I will give the name that I prefer as to the capacity, and any alternate name for the capacity that eventually comes up in philosophical literature. There is also much to be said on capacities and this article is only a basic treatment of these capacities. But this should give you a basic understanding of what capacities are and how they relate to the abortion issue and the discussion of human personhood.
Active vs. Passive capacities
Strictly speaking, an active capacity is a capacity that an entity has to develop something that is within its nature to develop. You have the capacity to read this article, and you still had this capacity while you were a human embryo (more on that below). Even though you couldn't read this article at the time when you were an embryo, you had it within yourself to develop that ability. Active capacities are identity-preserving capacities; in other words, everything that you have within yourself to develop you develop without becoming a different person. So you are the same person now as you were as an adolescent, a toddler, a fetus, and an embryo.
A passive capacity (sometimes called a compositional capacity) is the capacity for something to become part of the identity of something else, thereby losing its own identity. A passive capacity is not identity-preserving. For example, an oak sapling has the active potential to become an oak tree, but it has a passive potential to become a desk or a house (thereby no longer being a tree).
Sometimes active and passive capacities are understood as identity-preserving capacities, and referring to the passive potentiality as explained above by the term "compositional capacity," but the context of the passage should make it clear which sense of "active" and "passive" are intended. DiSilvestro in the same book (p.19) refers to an active potential as a potential to change things, whether in the world or in the bearer of the capacity. An example he uses is the act of standing. You remain the same person when you stand as when you sit. A passive identity-preserving capacity can be understood as powers for being changed by things in the world or in the bearer of the capacity. An example of a passive capacity in this respect would be the capacity for something oily to be burnt. An example of a compositional capacity (a passive capacity as illustrated in the previous paragraph) would be a lump of bronze being made into a statue. The statue is not identical to the lump of bronze it came from.
So all of the abilities that the zygote will develop are present from the very beginning, albeit not the ability to perform them. Any potential that an entity has within itself to develop is an identity-preserving capacity, which is why you are identical with the zygote that was conceived in your mother's womb, even though there are obvious differences between the two. But in what sense can the human zygote be said to have these abilities?
Inherent vs. Presently-Exercisable capacities
This simply refers to a hierarchy of capacities. You currently have the ability to read this article, to think, to sing (however off-key), to stand, to dance (a capacity woefully lacking in Caucasians like myself), etc.
An inherent capacity is a capacity that one has within oneself to develop. So whether or not you have developed these capacities, if entities of your kind naturally develop these abilities, then all entities of that kind have that capacity inherently. It's not until you actualize the potentiality that it moves from being merely an inherent capacity to a presently-exercisable capacity, and you will only actualize capacities that are within your nature to actualize (human beings will not develop the capacity to fly because it is not in their nature, nor will birds develop the capacity to write an opera).
I prefer the terms "inherent" and "presently-exercisable," or "immediately-exercisable," because it is obvious just in the names what that capacity entails, but capacities exist in a hierarchy. So you will sometimes see them referred to as second-order capacities (inherent capacities) and first-order capacities (presently-exercisable capacities). Or you will see them referred to as lower-order capacities, which is a presently-exercisable capacity, and higher-order capacities, which is just the capacity to obtain a lower-order capacity. DiSilvestro gives an example in his book (p. 20):
Liquid water has the lower-order capacity to evaporate, while ice has the higher-order capacity to obtain this lower-order capacity. An oak tree has the lower-order capacity to support a tree house, while an oak sapling has a higher-order capacity to obtain this lower-order capacity.Sometimes philosophers, like Eric Olson or Michael Tooley, speak of blocked or suppressed capacities. An example that Michael Tooley, a pro-choice atheist, uses in his book Abortion and Infanticide is about "Mary," who has the capacity to run a five-minute mile. If she gets drunk or gets out of shape, she would still have the capacity but it would become blocked or suppressed by external factors. But if you are speaking in terms of first- and second-order capacities, then suppressed capacities can be understood as second-order capacities.
So this naturally leads to the question of severely disabled human beings, such as anencephalic fetuses. According to J.P. Moreland and Scott B. Rae, in their book Body and Soul (InterVarsity Press, Downers Grove, IL, 2000, p. 72),
Higher-order capacities are realized by the development of lower-order capacities under them. An acorn has the ultimate capacity to draw nourishment from the soil, but this can be actualized and unfolded only by developing the lower capacity to have a root system, then by developing the still lower capacities of the root system, and so on. When a substance has a defect (e.g. a child is colorblind), it does not lose its ultimate capacities. Rather, it lacks some lower-order capacity it needs for the ultimate capacities to be developed.So in the case of an anencephalic fetus, they are still persons because they still have the same inherent capacities as other human beings, they are just being prevented from actualizing them due to external factors. A hundred years ago, someone who lost their sight would have lost it permanently. But today we have cornea transplants and other methods of restoring sight so that it is now possible for someone to regain their sight. This is because it went from being a first-order, immediately exercisable capacity to a second-order, inherent capacity. But the capacity was never lost, just like it was never lost a hundred years ago when it was impossible to restore it. So anencephalic humans are still human persons.
So this is what is meant when we talk about human nature, and the capacities (inherent and otherwise) that an entity possesses by virtue of the kind of thing that it is. Human zygotes have the same capacities we do now, just at a different level in the hierarchy. But these capacities they have are identity-preserving capacities, so they remain the same person throughout all of these changes. You are the same person now as you were in your mother's womb, right from the very beginning.