Thomas Aquinas, who is arguably a greater Aristotelian philosopher not only than Avicenna and Averroes but also than Mortimer J. Adler and Martha C. Nussbaum, thought that the moment of ensoulment with the specifically human soul occurs at the moment of birth (assuming live birth). Moreover, in the revolutionary 1973 ruling that legalized abortion in the United States, the Supreme Court of the United States established viability (i.e., the ability of the human fetus to live independently outside the mother's womb) as the legal standard. For all practical purposes, viability can be understood to be consistent with Aquinas's reasoning regarding live birth. Consequently, legal abortion in the first trimester does not involve "murder," a term that is used somewhat indiscriminately by some Christian antiabortion subversives. For no specifically human soul = no human life = no murder (i.e., the deliberate taking of an innocent human life). Opponents of legalized abortion in the United States today want to impose their mistaken and misguided moral values on other people instead of allowing those people to make their own moral choices. But render unto God what is God's due in churches and synagogues and mosques and other places of religious worship. But keep religious views out of the public square because of the doctrine of the separation of church and state.
Now, because of serious mistakes in philosophic reasoning, recent popes and yeasty Christian antiabortion subversives in the United States have stirred up strong irrational passions in the United States about abortion. However, the orthodox Christian religion as such is based on false and irrational claims, so perhaps it is not surprising that yeasty conservative Christians have been fomenting with such irrational passions about abortion, because oftentimes irrationality in one part of one's psyche can lead to irrationality in another part of one's psyche. We need to get clear about the irrationality of Christianity. As is well known, the historical Jesus was not a Christian, but a Jew. Moreover, the historical Jesus was not God, but an ordinary human being. He had a human nature, but he did not also have a divine nature -- that's an irrational absurdity. In addition, the supposed virgin birth of Jesus is an absurdity based on the translation of the Hebrew word for "young woman" (alma) into Greek in the Septuagint translation of ancient Hebrew scriptures that the various followers of Jesus read. It is another irrational absurdity for orthodox Christianity to claim that the historical Jesus was somehow born to a virgin. Furthermore, the supposed resurrection of Jesus from the dead involved grief-driven apparitions or hallucinations. Such apparitions or hallucinations involved the use of the psychological process that is involved when we dream when we are asleep, except that apparition experiences occur when we are awake. When we are awake and able to activate the psychological process that is involved in dreaming when we are asleep, C. G. Jung refers to such activation as active imagination -- see Jung on Active Imagination, edited by Joan Chodorow (Princeton University Press, 1997). As we will note momentarily, we can put active imagination to good use in the kind of prayer known in the Christian tradition as meditation. But the claim that Jesus somehow underwent resurrection is another irrational absurdity of orthodox Christianity. Finally, the idea of the divine trinity is another irrational absurdity. Because of all the irrational claims that many orthodox conservative Christians today believe, it is not surprising that yeasty conservative Christian antiabortion subversives have strong irrational passions about legalized abortion.
However, as I just intimated in passing, imaginative Christians today can still use the resources of their imaginations to engage in what Jung refers to as active imagination by doing the so-called spiritual exercises in the Spiritual Exercises of Ignatius Loyola, the founder of the religious order in the Roman Catholic Church known as the Society of Jesus (a.k.a. the Jesuit order). In this way, imaginative Christians today can learn to draw on the energies of the archetypes of maturity at the archetypal level of the human psyche, as Robert Moore and Douglas Gillette have described these in five books, most notably in the revised and expanded edition of The King Within: Accessing the King [Archetype] in the Male Psyche (Exploration Press, 2007). Even though Moore and Gillette happen to center their attention on the masculine forms of the archetypes of maturity in the male psyche, there are corresponding feminine forms of the archetypes of maturity in the female psyche.
In the present essay, I do not claim to have developed an argument from authority, but to have articulated a view of abortion that has not previously been articulated in exactly this by anyone else, as far as I know. However, ensoulment has been widely discussed -- the Wikipedia entry about ensoulment.
Finally, a personal note is in order. I was born because my mother deliberately declined to have an abortion when the medical doctor who told her that she was pregnant with me also told her that he could arrange for her to have an abortion, even though abortion was illegal at the time in the United States. As a result of the circumstances of my own birth, I want all women in the United States today to have the choice that my mother had when she learned that she was pregnant with me, as women in the United States have had since the revolutionary ruling in 1973 by the Supreme Court of the United States that legalized abortion in the United States. For the moral merit and the moral demerit (i.e., culpability) of our choices in life are proportionate to the degree of freedom that we have at the time when we make our choices. When the laws of the United States prohibited abortion, then compliance with those laws arguably reduced the degree of freedom involved, thereby moving the decision not to have an abortion into the domain of a pre-moral act. In Healing the Shame That Binds You (Health Communications, 2005), John Bradshaw has suggested that the domain of pre-moral acts is probably very large as a result of the inner psychological conditions such as the toxic shame that he discusses that limit the degree to which we are truly free to act.