By the embryologists from the moment the spermatozoön joins the nucleus of the ovum until the end of the second week of gestation the product of conception is called the Ovum; from the end of the second week to the end of the fourth week it is the Embryo; from the end of the fourth week to birth it is the Fetus. At what moment during these three stages does the human soul, the substantial form of a man in the full comprehension of the term, enter the product of conception? When does the thing become a human being?
The question is evidently one of the greatest importance. If the rational soul does not enter until the ovum has developed into an embryo, or only after the embryo has passed on into the fetal condition, the destruction of this ovum, by artificial abortion or otherwise, would be a very different act morally from such destruction after the soul had turned the new growth into a living man. If the product of conception has first only a vegetative vital principle, and this is later replaced by a vital principle that is merely sensitive, and this again is finally superseded by a rational vital principle, the destruction by abortion or otherwise of the vegetative or sensitive life would not be a destruction of a rational life. In this hypothesis the killing of the embryo would be a great crime, because the embryo would be in potency for the reception of human life, but the act would not be murder.
The discussion concerning the moment the human soul enters the body is older than Christianity, and it was taken up by many of the early Greek and Latin Fathers of the Church, and revived again and again down to the present day. Plato thought the soul enters at birth; Asclepias, Heraclites, and the Stoics held it is not infused until the time of puberty; Aristotle[15] said the soul is infused in the male fetus about the fortieth day after conception, and into the female fetus about the eightieth day.
Tertullian,[16] Apollinaris, and a few others advocated Traducianism,[17] or a transmission of the spiritual soul by the parents. He said souls are carried over by conception and by the parents, so that the soul of the father is the soul of the son, and from one man comes the whole overflow of souls. St. Augustine used the metaphor, one soul lit from another as flame from flame, without decay in either. Augustine was in doubt as to the origin of the soul, and inclined to traducianism, because it seemed to him better to explain the doctrine of the transmission of original sin. "Tell me," he wrote to St. Jerome in 415,[18] "if souls are created singly for each person born to-day, when do infants sin so that they need remission in the sacrament of Christ, sin in Adam from whom the flesh of sin is propagated?... Since we cannot say that God makes of souls sinners, or punishes the innocent, nor may we hold that souls even of infants which without baptism leave the body are saved, I ask you how that opinion can be defended which thinks that all souls are not made from the single soul of the first man, yet as that soul was one to one man, these are particular to particular individuals."
Again, St. Augustine said:[19] "I do not know how the soul came into my body; he knows who gave it, whether he drew it [traxerit] from my father, or created it new as in the first man." In the Book of Retractions,[20] speaking of the articles he had written against the Academicians before he was a bishop, he says: "As to the origin of the soul, how it is set in the body—whether it is from that one man who first was created ... or, as in his case, is made particularly for each particular individual, I did not then know, and I do not know now." St. Gregory the Great also said he could not tell whether the human soul descends from Adam or is given particularly to each man.
St. Gregory of Nyssa, however, who died about 385, thirty years before St. Augustine wrote the letter to St. Jerome, held that the soul is infused into the body at the moment of conception, and he argues with absolute precision for his opinion.[21] St. Maximus the Theologian, who was martyred in 662, inveighs[22] against the notion that the soul is vegetative at first, then sensitive, and finally intellectual, and he thinks the assertion of Aristotle that the fetus is not animated before the fortieth day is altogether untrue.
St. Anselm, who died in 1109, very dogmatically denied that the fetus is animated at conception,[23] and after his time the doctrine of Aristotle, which is commonly called the Thomistic opinion, became almost general. Vincent of Beauvais, however, a contemporary of St. Thomas, opposed the Thomistic doctrine. Albertus Magnus[24] had the same opinion as St. Thomas, and probably taught it to St. Thomas. In the middle ages all held that each soul is directly created by God, and is infused into the embryo, not at the instant of conception, but when the embryo is sufficiently formed to receive it, which, as Aristotle said, happens at about the fortieth day in males and the eightieth day in females. The Thomists maintained the succession of the three souls; many others opposed this particular opinion.
Thomas Fienus, a physician and a professor in the University of Louvain, in 1620 published a book[25] in which he held that the soul is infused about the third day after conception, and his argument for the early advent of the soul is very sound. As a result of Fienus's revolutionary argument, Florentinus in 1658 brought out a book at Lyons, called De Hominibus Dubiis Baptizandis, in which he held that no matter what the age of the aborted fetus, if it could be differentiated from a mole it should be baptized. This book was brought before the Congregation of the Index. The congregation did not condemn the book, but the author was forbidden to teach that his doctrine holds sub gravi. The book went through many editions and was approved by the faculties of the principal universities and the theologians of the leading religious orders.
Zacchias, chief physician to Innocent X., in 1661 published his Questiones Medico-Legales, and in this he maintained that "the human fetus has not at any time any kind of soul other than a rational, and this is created by God at the first moment of conception, and is then infused."[26] By 1745 the opinion of Zacchias as to the moment life begins was virtually general among physicians, and has since remained the doctrine of physicists. Modern discoveries by biologists have confirmed the fact that human life exists in the impregnated ovum exactly as it does in all stages of life, and no scientist holds any other opinion. There are, however, a few moralists at the present day who incline to the old Thomistic doctrine or to modifications of it.
St. Alphonsus Liguori[27] was a follower of the Thomistic opinion. He affirmed: "They are wrong that say the fetus is animated at the instant of conception, because the fetus certainly is not animated before it is formed, as is proved from Exod. xxi: 22, where in the Septuagint version we find: 'He that strikes a gravid woman and causes abortion, will give life for life if the child was formed; if it was not formed, he will be fined.'" This argument by St. Alphonsus is invalid apart from any facts that may bear upon either the Thomistic or the modern opinion concerning the quickening of the fetus. The text from the Septuagint Exodus is (1) too doubtful in itself to be the basis of any argument; but (2) even if it were authentic just as it stands, the conclusion St. Alphonsus draws from it is not warranted by the premises. The Septuagint text differs from the Vulgate and the Hebrew texts. The Vulgate has it thus: "Si rixati fuerint viri et percusserit quis mulierem praegnantem, et abortum quidem fercerit, sed ipsa vixerit, subjacebit damno quantum maritus mulieris expetierit et arbitri judicaverint; sin autem mors fuerit subsecuta, reddit animam pro anima, oculum pro oculo, dentem pro dente, manum pro manu, pedem pro pede, adustionem pro adustione, vulnus pro vulnere, livorem pro livore."[28] This version has nothing whatever to say about the foetus formatus or non formatus; it is merely an application of the Semitic Lex Talionis, and the form of the law is clearly corrupt and inaccurate.