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vital principle, and this is later replaced by a vital principle that is merely sensitive, and this again is finally superseded by a rational vital principle, the destruction by abortion or otherwise of the vegetative or sensitive life would not be a destruction of a rational life.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. [15] _IX, De Animalibus_. 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." [16] _De Anima_, cap. 27. [17] From _tradux_, a planted vine-shoot made to take root. [18] Migne, vol. xxxiii, col. 720.