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 according to Frank Flinn, author of the Encyclopedia of Catholicism who teaches at Washington University in St. Louis.
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.
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 Three years later Gregory XIV rescinded the severity of Sixtus' punishments and reinstated the doctrine of delayed hominization or "quickening" of the fetus, approximately sixteen weeks after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.
More Roman Catholic commentary on the election and abortion here.