A prime focus abortion debate is the timing of "ensoulment," that is, when the soul enters the human embryo and makes it fully human. The language of this issue is a great obstacle, because there are two notions of the soul easily confused.
The more common understanding of "soul" is the invisible, supernatural part of the human being, the spirit, that will survive our bodies. Being wholly immaterial and most evident in intellectual acts, it is understandably difficult to point to evidence of its presence in any but a conscious, fully formed human.
The concept of soul more relevant to abortion is that of the natural form of the body: what holds its parts together and animates it. Unlike the spiritual soul, this natural soul is manifestly evident from the first moment of conception, and historically the law has been quick to recognize the rights of the unborn thus ensouled. Chapter 8 of the NCBC's Handbook on Critical Life Issues documents the evolution of laws regarding the unborn through the centuries as medical knowledge of life in utero has grown.
Already in 1823, a standard American work on law, Elements of Medical Jurisprudence, by Theodore Beck and John Beck, argued for considering "vitality" from the moment of conception on grounds of reason and physiology....
In 1803, the first British statute against abortion condemned as a felony any attempt to procure an abortion [and] cleared up any confusion in common law about the act of abortion before quickening: it was a felony.
The significance of quickening, the point at which the mother can feel the child stir in her womb, is that it was regarded as the time when the soul entered the developing body. In the 18th century, Blackstone had set it at the point at which the fetus was to be regarded as living legally.
But the advances of medical science have clearly demonstrated that the child is ensouled from conception. It is not the mother's body that assembles the embryonic body; she only delivers the nutrients that the embryo itself arranges into itself. Certainly, in the earliest stages of zygotic development, the cellular machinery from the egg's cytoplasm plays an essential role in "jumpstarting" the process, but the force of the new life is itself indispensibly central.
Science refutes the old assumption that the fetal soul enters an already assembled body. Rather the soul is present from the beginning and is the primary architect of its body. We of the modern age take too much for granted modern science's revelation of natural wonders. The reason previous ages didn't criminalize abortion throughout pregnancy is that they had no idea how early the natural soul of the developing child manifests its presence.
We who seek to defend that life need to be clear that we base our defence of unborn humanity not by pulling some sort of invisible, supernatural spirit out of rhetorical thin air, but by simply following the evidence that modern science presents to us so clearly.
John A. Leies, Donald G. McCarthy and Edward J. Bayer, Handbook on Critical Life Issues, ed. Louise A. Mitchell (Boston: National Catholic Bioethics Center, 2004), ch. 8. [The book itself is burdened by an easily misunderstood definition of the human being]
Also: Robert P. George, "Statement(joined by Dr. Gomez-Lobo)" in President's Bioethics Council, Human Cloning and Human Dignity (Washington, D.C.: 2002), 258-266.