Article ID: JAF3403 | By: Clinton Wilcox

This article first appeared in the Christian Research Journal, volume 40, number 3 (2017). The full text of this article in PDF format can be obtained by clicking here. For further information or to subscribe to the Christian Research Journal go to: 


People use the word “potential” in many different ways. A child can show potential when it comes to having sports or musical aptitude. If a speaker gives a presentation without being adequately prepared, there is the potential for disaster. However, in philosophy, potential is a technical term, and it is not as simple as one might expect. An oak sapling has the potential to become a mature oak tree, and it has the potential to become a house. But each of these senses of the word potential is different and has real-world application to debates in bioethics.

Some pro-choice advocates try to argue that human embryos are not persons but are “potential” persons. In this view, embryos are biological members of the human species, but they have not yet reached the developmental milestone necessary for grounding their identity as persons. However, calling the human embryo a “potential” person is misguided. A common idea is that living things are constructed piece by piece from the outside, when, in fact, they develop from within themselves. To say that a human embryo is a potential person is misguided because it fails to take into account the fact that the reason the embryo eventually will become an adult is because she is a human being with a human nature, and it is this nature that directs her development as a human being. As she has a rational, volitional nature, she will develop all of the present capacities to perform personal activities. Human embryos are not potential persons; they are actual persons with potential.

Attempting to mitigate their natural compunction about human embryo experimentation and abortion, pro-choice advocates often contend that human embryos (and fetuses) are not persons but are “potential” persons. Although embryos are biological members of the human species, they say, embryos have not yet reached the developmental milestone necessary for grounding their identity as persons. This idea is misguided, however.

In order to demonstrate the personhood of an embryo, it is necessary to grasp several scientifically informed and philosophically rich concepts — the notions of nature, capacity, substance, and potentiality. These concepts can be difficult to grasp, but through perseverance they can be understood. Mastery of these concepts is very important because understanding them will equip you to recognize why the more sophisticated arguments for abortion fail.


For purposes of this article, it is sufficient to say that a thing’s nature determines the kind of thing it is and orders its ultimate capacities so it can flourish according to its kind.1 To determine what kind of thing something is requires observation and experience. We know from observation that dogs bark, so it can be said that in the nature of canines is the capacity to bark.

A capacity can be defined as the metaphysical ground of a thing,2 such that a dog has the capacity to bark just because it is the sort of thing that barks. Humans, too, have a nature, and observation and experience show that in virtue of their nature, they have the capacity for rational thought — a human being just is the sort of thing that can think rationally. Philosophers commonly speak of capacities in terms of powers or dispositions.3 So to say that something with a human nature has the capacity for speech is just to say that humans have the ability and propensity to exhibit speech. If functioning as humans should, then, each person will speak. If a human fails to develop the ability for speech, she does not cease being human (since she is human in virtue of her nature), but her capacity for speech is blocked.

Capacities exist in hierarchies.4 I currently have the first order capacity (present capacity) to speak English. When I was an embryo, I did not. The capacity to speak English was a second-order capacity until I was able to learn and reproduce the words, understanding what I was speaking. Then it became a first-order capacity. I also currently have the second-order capacity to speak Italian, which will not become a first-order capacity unless I learn the language. Moreover, philosopher J. P. Moreland explains, “There are first-order capacities, second-order capacities to have these first-order capacities, and so on, until ultimate capacities are reached….A higher-order capacity is realized by the development of lower order capacities under it.”5 My first-order ability to speak English relies on lower-order capacities, including developing my lower capacity to have a larynx, mouth, and tongue. As my lower-order capacities develop, I fulfill my ultimate capacities and, in principle, flourish according to my human nature.

How an entity acquires these capacities is determined by the kind of thing it is — its nature. We know from science that living organisms such as horses and humans begin life at fertilization, and each entity’s capacities develop within and by the organism itself — living things have their capacities inherently (at the second-order level). Humans, then, exhibit rational thought, speak languages, create art, love one another, and so on, because by nature they possess these inherent, second-order capacities from conception. However, nonliving artifacts, such as cellos and computers, must first be constructed before they possess any of the capacities had by other things like them.

Artifacts vs. Substances

Confusion in the abortion issue stems from seeing unborn humans as less like living things that naturally develop and more like cars that are constructed on an assembly line. It is crucial, then, to understand the differences between artifacts and what philosophers call substances.6 An artifact, such as a car, is an artificially constructed thing and has no inherent capacities but gains capacities at its construction from outside itself. A substance is an entity that maintains its identity through change — artifacts do not maintain their identity through change, but all living things do. All living things are substances. To better apprehend these abstract ideas, consider levels of organization in nature.

Heap. A heap is the weakest kind of unity. A pile of scrap metal is a heap. A heap has unity only insofar as each item in the heap is spatially located near each other.

Artifact. An artifact is a stronger kind of unity than a heap but weaker than a substance. Its parts are not merely spatially close together, but it has a mechanical unity in the way its parts work together according to how its designer designed it. Among the many differences between artifacts and substances, two are crucial for this discussion.

The first difference to consider is that an artifact’s individual parts not only exist before its construction but also have metaphysical priority over the artifact. A car is not able to transport someone until all the parts are in place. A car does not have this capacity inherently, because it is constructed by an outside builder from a heap, a pile of metal. That pile of metal would not become a car on its own; that metal could become anything, such as a boat, a house, or Christmas ornaments. Also, the parts are what determine what the car does.

The second difference to consider is that if one exchanges a part on an artifact, the artifact is not, strictly speaking, the same entity it was before. This is why cars lose their value to collectors if any part of the car has been changed, even if just the hood ornament.

Substance. A substance is the strongest kind of unity. It has an internal unity that directs its own development, and thus living organisms constitute paradigm cases of substances. As such, unlike artifacts, it is the substance’s nature that determines its parts and capacities. Whereas an artifact derives its unity from its parts, a substance derives its unity from its internal nature. This is why a human embryo is a human being once it comes into existence at fertilization. It is biologically human, and it has a human nature. And, again, unlike artifacts, a substance maintains its identity through any changes it undergoes that do not kill the substance. If I lose my arm in an accident, I am still “me” despite losing one of my parts.


This leads, finally, to a discussion of potentiality. There are different kinds of potential. An oak tree sapling has the potential to become a mature oak tree, but it also has the potential to become a house. A pile of scrap metal has the potential to become a car, but it also has the potential to become a boat. As an embryo, I had the potential to speak English, but I also had the potential to speak Italian, only one of those potentialities thus far having been actualized.

Active and Passive Potential

Aristotle, in his book Metaphysics, made a distinction between active and passive potentials.7 An active potential’s locus is in the agent itself (e.g., where heat or the art of building are present, the active potential to produce heat is present in the thing that can produce heat, and the active potential to build is present in the man who can build). A passive potential is a potential that is in the thing acted upon (e.g., whatever is oily can be burned, and whatever yields in a particular way can be crushed). Philosopher Russell DiSilvestro puts it this way: “‘Active’ capacities are powers to change things, whether in the world or in the bearer of the capacity, while ‘passive’ capacities are powers for being changed by things in the world or in the bearer of the capacity.”8

So consider a tree sapling. An oak tree sapling has the active potential to become a mature oak tree. This is because it has the potential within itself to develop into a mature oak tree, and such developmental changes preserve its identity as an oak tree. However, that tree sapling also has the potential to become a house. It has this potential passively because it must be acted on from outside itself in order to become a house — and being so acted on, its identity as an oak tree is no longer preserved.

We say that an active potential is an identity-preserving potential because it is within the substance’s internal program ming to undergo that change, but also because there is a further distinction to be made. This further distinction concerns various meanings of the word “is.” There is a difference between the “is” of identity and the “is” of constitution. To say Clark Kent is Superman and Clark Kent is Kryptonian is to use the word “is” in two different ways: the first is to speak of Kent’s identity, and the second is to speak of his constitution, or what goes in to make Kent what he is. Furthermore, DiSilvestro points out, there is a difference between what one might call the “is potentially” of identity and the “is potentially” of constitution.9 I am sitting at my desk while writing this article. My capacity to stand means that if I were to stand up, I would remain the same person I was while I was sitting down. This is the “is potentially” of identity, because while I am sitting, I potentially can be standing and will remain the same individual. So one can say with this sense in mind, Wilcox sitting is potentially Wilcox standing. However, to return to the example of the car, the hunk of metal’s capacity to become a car does not imply it will retain its numerical identity when it goes into constructing a car. This is the “is potentially” of constitution, because a hunk of metal can potentially become a car, but it will not retain its identity through being made into the car. So in this sense the pile of metal is potentially a car.

Finally, among the many remaining distinctions concerning potentialities that could be drawn, the following one is relevant for this discussion. Humans develop their capacities over time, like all living things, but as rational animals (Aristotle’s term), humans develop some capacities involuntarily and others voluntarily. The capacity for consciousness is a capacity that naturally develops in virtue of human nature. The capacity to speak English is not. Humans have the inherent capacity to speak a language, but not the inherent capacity to speak English, since English is socially constructed. So while I did not voluntarily develop the capacity to speak English (it developed in me because I grew up in an English-speaking culture), my capacity to speak Italian is one I must voluntarily develop (and the capacity to speak English is one a native Italian speaker must voluntarily develop). So my capacity to speak English or Italian is rooted in the inherent human capacity to speak a language. Think of this distinction as natural versus acquired capacities.10


By now it should be obvious what this discussion has to do with the abortion issue. The human embryo has the active potential to develop personal properties, such as consciousness, rationality, and the ability to communicate via language. Since this active potential is rooted in a person’s nature as a rational animal, a human embryo is thus a rational entity by nature. It does not have the presently exercisable (i.e., first-order) capacity to be rational, but it has the inherent (i.e., second-order) capacity to be rational, which is just a capacity to get the present capacity for rationality. So even though a human embryo is not presently rational, it is right to say the embryo is rational because it is rational by nature, just like it is right to say I am a rational entity while I am asleep, even though during sleep I am not presently able to exercise that capacity (due to being unconscious).

Furthermore, the reason the embryo eventually will become an adult is that she is a human being with a human nature, and it is this nature that directs her development as a human being; moreover, it is in virtue of this nature that she retains her identity as a rational person through change. Few would deny that I am the same individual I was ten years ago, even though I have undergone change. By the same reasoning, I am thus the same individual I was when I was an embryo; and, as such, if it is wrong to kill me now, it was wrong to kill me then.

Singer: Is a Potential X an Actual X?

Consider now some possible objections to this thesis that human embryos are rational persons worthy of protection. Bioethicist Peter Singer offers the following statements as counterexamples to the idea that a potential X has the same value as X, or has all the rights of X: “To pull out a sprouting acorn is not the same as cutting down a venerable oak. To drop a live chicken into a pot of boiling water would be much worse than doing the same to an egg. Prince Charles is a potential King of England, but he does not now have the rights of a king.”11

Do Singer’s examples counter the thesis that human embryos are rational persons worthy of protection? Not at all. As I have shown, the sprouting acorn is the same entity as the mature oak tree it will one day become, even though one might indeed value the mature oak tree over the sprouting acorn. Singer’s analogy introduces the notion of intrinsic value. If an entity has intrinsic value, its value is dependent on what it is — its value comes from within itself and not from what it can do for others. As a rational, volitional person, a human being possesses intrinsic value and is thus always valuable. Plants and trees, however, are not valuable in and of themselves. Their value comes from outside themselves. They instrumentally are valuable only insofar as persons, say, need them to take in carbon dioxide and produce oxygen, to provide shade, or to use them for materials. The instrumental value of trees changes in accordance with their usefulness to intrinsically valuable persons. So it is incorrect to think one is not cutting down the same entity, but there are other considerations besides potential that make it a different matter to cut down a full blown tree than to uproot a sprouting acorn.

A chicken comes from an egg, but it is not the egg itself. And chickens, like trees, are not intrinsically valuable but are instrumentally valuable. They are valuable only insofar as a person, say, takes pleasure in raising them or has need to consume them for food.

Finally, Prince Charles does not have the rights of a king despite being a potential king because he is not a king by nature. There are conditions that must be satisfied before he can ascend to the British throne, such as the death of his mother, the reigning queen. But humans are rational by nature, and it is this rational nature that grounds a human’s intrinsic value. Humans are never potential persons; they are actual persons with potential.

Tooley: Kitten Intelligence Serum

Philosopher Michael Tooley offers the following thought experiment as a counterargument to the idea that killing potential persons is intrinsically wrong: Suppose a scientist develops an intelligence-causing serum that can cause a cat to develop all the properties we ordinarily associate with persons (e.g., consciousness, self-awareness, the ability to use language). Now suppose we have two kittens, giving the serum to one kitten but not to the other. Suppose we then give a “neutralizing” agent to the kitten we gave the serum to before it completes the intelligence-gaining process. It seems the action of giving the neutralizing agent to the first cat and not giving the intelligence serum to the second one at all are morally the same. So this shows that we are not obligated to complete the process of personhood once it is begun.12

The idea that Tooley is arguing for is that if we were to inject an intelligence serum into a kitten causing it to develop first-order capacities that make it a person, we would be morally obligated to protect and not kill the cat person. However, it doesn’t follow from this fact that if we begin the process to turn the cat into a person, we then would be obligated to let that process finish.

Tooley’s thought experiment assumes that unborn humans are potential persons, which, as I have shown, is false. Human embryos are actual persons with potential. Also, like Singer, he conflates two different senses of potential. If we were to create an intelligence serum for kittens, we could say they have the potential to be persons. But they would have this potential passively — one must act on them from the outside in order to make them persons. They do not have this potential actively to develop themselves. If they had the potential actively, then we would be obligated to let the process finish. But cats are not persons by nature. Since we have no obligation to turn them into persons, we also have no obligation to finish the intelligence giving process once we begin it. But once a new human being has been created through the procreative act of sex, we have an actual person, and we are now obligated to allow that person to develop according to her nature.

Allow me to reiterate that since the human embryo is a rational entity from fertilization, and all changes it will undergo are within its internal programming (its nature) to undergo, all its changes are identity-preserving. So I was the same entity as an embryo that I am now. Since it is seriously wrong to kill me now, it was wrong to kill me then. Since all human embryos share the same human nature, it is always seriously wrong to kill a human embryo without very strong justification.

Clinton Wilcox is a staff apologist for Life Training Institute and a certified speaker and mentor for Justice for All. He specializes in training pro-life people to make the pro-life case more effectively and persuasively.


  1. I owe Scott Klusendorf for this definition. While it is beyond the scope of this article to give a full defense of natures, for a book that does so, see Edward Feser, Scholastic Metaphysics (Neunkirchen-Seelscheid, Germany: Editiones Scholasticae, 2014).
  2. Russell DiSilvestro, Human Capacities and Moral Status (New York: Springer, 2010), 17.
  3. Ibid.
  4. William Lane Craig and J. P. Moreland, Philosophical Foundations for a Christian Worldview (Downers Grove, IL: InterVarsity Press, 2003), 216–17.
  5. Ibid.
  6. I’m using Philosophical Foundations for a Christian Worldview by Craig and Moreland as a guide for this section. See their book for more on these differences.
  7. For this discussion, I draw from and paraphrase DiSilvestro, Human Capacities and Moral Status, 19.
  8. Ibid.
  9. Ibid.
  10. Special thanks to Timothy Hsiao for his thoughts on this paragraph in personal correspondence.
  11. Peter Singer, Practical Ethics (New York: Cambridge University Press, 1993, 1997), 153.
  12. Michael Tooley, Abortion and Infanticide (New York: Oxford University Press, 1983), 191, paraphrased.