Here’s a real eyebrow-raiser: the author of the Encyclopedia of Catholicism, Dr. Frank Finn, says that on the subject of abortion and church teaching, Joe Biden has a point:

Democratic vice presidential candidate Joseph Biden is telling the Catholics in his audiences that St. Thomas Aquinas had a different teaching on abortion than the current pope and his immediate predecessors. Many Catholics are saying, “He simply cannot be right.” Well, the short answer is: Biden is right. The news media are saying that American bishops are giving him a theology lesson on abortion. Mr. Biden is in a position to give them one right back.

The Catholic teaching on abortion has complex roots in Jewish teaching, Greek thought and early Christian doctrine. Jewish teaching shows great reverence for life as a gift from God. The law of compensation in Exodus 21:22 makes a distinction between the penalty for striking a pregnant woman that ends in the loss of the fetus (a monetary amount) or the mother (death).

The Greek Septuagint text of this verse shows the influence of Greek thought. It distinguishes between incompletely and completely formed fetuses, and exacts a penalty of death in the case of the abortion of the latter. This is a clear reference to Aristotle’s distinction between three types of souls corresponding to three types of living beings: plants, animals and humans. Aristotle taught that the human fetus does not receive a human soul until it takes on a human form. This became known as the delayed hominization thesis or the late implanting of the human soul.

Greeks and Romans were not squeamish about abortion. They held the state to be supreme and abortion was permissible if political reasons dictate. Thus they freely used drugs causing abortions and exposed defective children, even normal female children if a male heir was desired first.

Like their Jewish contemporaries, second century Christian communities rejected abortion. Many early Christian writings, like the Didache and the Epistle to Barnabas, preach against abortion. Several early local councils of bishops condemned abortion which they thought was closely linked with adultery. Augustine, however, affirmed the Aristotelian teaching in his Commentary on Exodus and stated that he did not have an answer about the status of a fetus.

The abortion question entered into a new phase during the Middle Ages. St. Anselm of Canterbury gave the most forceful statement in favor of the delayed hominization thesis: “No human intellect accepts the view that an infant has the rational soul from the moment of conception.” St. Thomas Aquinas also accepted Aristotle’s theory of the late appearance of the human soul in the fetus. He taught that the abortion of a fetus before it has the human soul is a sin against marriage but that it is not murder. The famous medieval jurist Gratian wrote, “He is not a murderer who brings about abortion before the soul is in the body.”

It is important to note that for roughly 500 years the Catholic church followed the teaching of Aristotle and St. Thomas on the status of the fetus. The Council of Vienne (1312) under Pope Clement V affirmed Aristotle’s teaching on delayed hominization. But in 1588 Sixtus V issued the bull Effraenatum excommunicating anyone who used contraception and induced abortion at any time. Three years later Gregory XIV rescinded the severity of Sixtus’ punishments and reinstated the doctrine of delayed hominization or “quickening” of the fetus, approximately sixteen weeks after conception. This rule remained in effect for another three hundred years until 1869 when Pope Pius IX imposed automatic excommunication for abortion at any stage of pregnancy. Implicitly Pius’s teaching embraced a theory of the immediate implanting of the soul at the moment of conception.

This last position was reaffirmed at the Second Vatican Council in 1962. In 1987, the Congregation for the Doctrine of the Faith issued an instruction called DonumVitae which said that a fertilized egg would become nothing other than a human being. However, the earlier instruction Declaration on Procured Abortion (1974) expressly left aside the question of the moment when the spiritual soul is infused. Many thought that Donum Vitae slammed the door on Aristotle’s and Aquinas’s position on the fetus.

There’s much more at the link.

UPDATE: Sen. Biden’s bishop has a few thoughts on this subject — and, as you might expect, he strongly disagrees with the Senator from Delaware.