What’s at stake in the abortion debate is precisely what was at stake during the nineteenth-century debate about slavery: Where do we draw the line between beings we can simply use for our own purposes and discard when inconvenient, on the one hand, and beings who have fundamental rights, who deserve our respect and protection, on the other? Abraham Lincoln expressed the central point in the slavery debate: “If the Negro is a man, why then my ancient faith teaches me that ‘all men are created equal;’ and that there can be no moral right in connection with one man’s making a slave of another.” Likewise, if the human embryo or fetus is a man, a human being, then we owe him love and care, and protection from being ripped to pieces or discarded in a trashcan (the objection that a human embryo is a human being but not a person is considered here).

This is the argument that Samuel and Maureen Condic make in their new book, Human Embryos, Human Beings, A Scientific and Philosophical Approach. The Condics are a brother–sister team, he a trained philosopher and she a neurobiologist. They present a careful and detailed case for the proposition that a human being comes to be at fertilization, and refute the main arguments to the contrary. Along the way they clarify the concepts of substance, substantial form, soul, organism, and final and formal causality.

Development and Determinacy

The standard textbooks in embryology and developmental biology locate the beginning of the human individual at the union of the sperm and the egg. Still, there have been various philosophical attempts to avoid that conclusion.

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Here is the simplest objection: The embryo is only developing specifically human traits, but does not yet have them, and so is not yet a human being. The Condics’ reply sets out a central philosophical point: Organisms develop; they typically come to be long before they perform actions characteristic of mature individuals of their kind. In plants, for example, “the development of significant new structures (e.g., the development of bark around the ‘trunk’ of a seedling . . .) is not a sign of a new thing being generated (substantial change), but rather of an existing thing developing (accidental change).” The evidence for this point is clear: the plant regularly develops such structures, in the absence of an external organizing cause. A regular, predictable, and internally determined developmental trajectory is evidence that an organism is maturing toward the adult stage of individuals of its kind.

But perhaps the developmental pattern is not so clear? Others argue in various ways for indeterminacy in that development. They claim that the point in development where one marks the beginning of a human being involves a choice. Ronald Green argues: “Because biological realities involve [gradual or continuous] processes, the determination of significant points within these processes inevitably involves choice and decision on our part.”

However, a human organism is neither a quality (like the color red) inhering in something else, nor a mere aggregate (like a heap of sand)—both of which come in degrees and have indeterminate beginnings. Rather, a human organism is a substance, an entity that exists in itself rather than in another, with an inherent propensity toward acting and reacting in specific ways, and persists as a whole at each moment of its life cycle, though in differing stages of maturity. A particular individual either is a human organism or is not, there are no degrees—for example either you exist or you do not, however sick or near death you might be. And so a human organism comes to be at a particular time. Further, at fertilization, that is, when a male spermatozoon enters the female oocyte, each of those cells ceases to be, and a new organism, with a new developmental trajectory toward the adult stage of a human, comes to be.

Others advance an objection similar to Green’s, appealing to “developmental systems theory.” This theory rightly holds that DNA is not the sole determinant of an organism’s behavior, that other factors also help shape it. Some authors go further, however, and deny the existence of any line between what is internal to the organism—its determinate nature—and what is external. And so they claim that one cannot discern the time at which the human organism comes to be.

The Condics reply that from the fact that an organism’s survival and flourishing depends on extrinsic as well as intrinsic factors, it simply does not follow that organisms are merely indeterminate outcomes of converging forces. If a human zygote and a skin cell are placed in the same circumstances—a human womb—the human zygote will develop to a stage along a trajectory whose endpoint is a mature human, whereas the skin cell will behave quite differently. What will happen to each does depend in part on factors external to it, but it is plain that within each cell there is a distinctive tendency to act and react in a distinctive manner in relation to the given environment. That inherent orientation provides an objective standard for determining when it (he or she) comes to be.

Aquinas’s Delayed Hominization Theory

Some have appealed to St. Thomas Aquinas’s theory of “delayed hominization,” claiming that his basic argument is still valid. Aquinas held that the human being is a body-soul composite, that the soul is related to the body as substantial form to matter, and, as with other substantial forms, that the soul can be present only in matter proportionate to it. Only a body structured so that it is capable of participating in specifically human actions is proportionate to the human soul. Aquinas concluded that the human embryo is not a human being. Rather, the generation of a human being takes place in several steps, resulting first in an organism with only a vegetative soul, then after some development, is succeeded by an organism with a sensitive soul, and only after more development, by an organism with a rational soul, a human being.

The Condics agree with Aquinas’s proportionality principle: only a body capable of sharing in specifically human actions can have the human form. However, not all specifically human actions presuppose brain activity. In particular, the formation of a mature human body is also a specifically human action—and thus requires a human cause. Aquinas and Aristotle saw this; and so they held that the male semen—the active cause in generation in their view—was only the instrumental cause, and that the father, acting through the mediation of his semen, was the principal cause. Thus, in their view the semen had to remain present throughout the generative process, gradually forming the material provided by the mother, menstrual blood, until a body apt for having a rational soul was reached.

Of course we now know, the Condics point out, that nothing in the male semen remains as a distinct agent in the process of the embryo’s development. After fertilization neither the male sex cell nor the female sex cell remains—and the contributions from the mother subsequent to fertilization are not formative but provide only nutrition and a suitable environment. Aquinas and Aristotle were right that a proportionate cause of embryogenesis is needed; but that cause is the embryo itself. The embryo is the principal cause of its own development to the mature stage of a human being. Thus, when combined with the embryological facts known today, Aquinas’s basic proportionality principle entails that the human embryo is a human being, though at an immature stage.

Individuation and DNA

Some have objected to the position that the embryo is a human being on the basis of the apparent totipotency of the cells within it. In the first few days the embryo’s cells seem to be totipotent—that is, the embryo’s cells seem capable of forming a completely new embryo if separated from the rest. So (the objection is) if the potentiality to develop to a human adult shows that an entity is a human being, it will follow that the early embryo is a multitude rather than an individual—and so not a distinct individual. The Condics reply: The potentiality to be divided is remote (requiring intermediary steps) and passive (a potential to undergo a change from another, rather than a potential to do something). Now, there is nothing incoherent in an organism’s being an actual individual and at the same time having the remote and passive potential to be dividedeven into two individuals of the same species. Flatworms and plants from which cuttings can be obtained are two examples. The objection from monozygotic twinning is similarly dealt with: such twinning is probably a form of budding—embryo A splits into B and C, but one of those (B or C) is identical with A—there is nothing logically or metaphysically incoherent with that.

It has also been objected that a human organism does not come to be until the DNA contained in the chromosomes of the embryo begins to guide the developments of the embryo (at about three days after fertilization). Between day 1 and day 3 the embryo’s activities are “driven,” (the objections goes) by the maternal RNA rather than by the zygotic (the embryo’s) DNA. Not until that point, it is objected, is the embryo a distinct individual.

However, in the first place, so-called “maternal RNA” is at this point actually part of the embryo herself, and so is more properly termed “maternally-derived RNA.” These genetic materials are contained within the developing embryo, not extrinsic to it; moreover, they interact with, and are regulated by, other components in the embryo, including proteins derived from the sperm. The maternal RNA molecules are organs of the organism as a whole, not external agents “driving” its development. Thus, the agent performing the directed development is the embryo itself rather than any external agents. Also, the Condics point out that the activation of the zygotic genome is in fact initiated within the first ten hours after sperm-egg fusion—much earlier than this objection supposes.

To support the previous objection, some argue that there must be a single overarching structure to guide development through the whole of the organism’s life, and an organizing structure is not present until the zygotic genome becomes active. This argument, too, is flawed. For one thing, each cell within the embryo has its own DNA. If the organism must be controlled by a single material part, it’s not clear which of the hundreds or trillions of instances of that genome does that job. Conversely, some individuals—mosaics—have more than one type of genome. Thus, while organisms often have a “central controlling part,” at least during long stages of their lives, it is not an a priori necessity. What’s necessary is that the several parts of an organism constitute a true substance, not a mere aggregate, that the actions are explained by the nature of the whole, not wholly by the natures of the parts.

Thus, the maternally derived RNA molecules are parts of the embryo, and provide only part of the developmental program present within the embryo. With the union of the spermatozoon and the oocyte a distinct organism is constituted that has within itself the entire developmental program (which includes the maternally-derived DNA, as well as other elements derived from both the maternal and paternal contributions in fertilization).

The Overall Argument Summarized and Illustrated

The Condics illustrate the overall argument by comparing embryogenesis to a bridge in the process of being built—both the dissimilarities and the similarities are instructive.

We understand what a completed bridge is by its organization; each part contributes to the function of the whole. We understand what a bridge under construction is by its anticipation of that completed form. These parts are assembled, in this particular order and arrangement, because they are needed to produce a completed bridge. Similarly, the activities of the embryo make sense in their location along a developmental path whose endpoint is a mature human body. These parts are assembled, and in this particular order and arrangement, because they are needed (or because of their effectiveness) to form a mature human body. The human embryonic path of development is intelligible precisely as having the endpoint it does—a mature human being. Events occurring even in the first hours and days are needed to ensure that a human face rather than a baboon one will be produced, or that a human brain, rather than another kind, will be produced. These points provide evidence that the embryo has within it an inherent orientation to the mature stage of a human being, and so is a human being.

The difference between the bridge under construction and the embryo is equally important. The plan for the bridge is extrinsic to it, which is why in a sense the incomplete bridge is not, or not yet, a bridge. However, the embryo is not being constructed from the outside by an external agent (this was the empirically refuted idea of Aristotle and Aquinas). The nearest external agent, the mother, provides only nutrition and a hospitable environment. The orderly and intelligible construction of the mature embryo’s body does need an explanation, but that explanation is the embryo itself.

In sum, the criterion for determining whether something is a human being at an early stage of development is: having the structure that provides an orientation toward the mature stage of the human being. The human embryo possesses that from fertilization onward; so the human embryo is a human being.