An article of our faith, which we are obliged to embrace, states, “Every human being possesses an individual soul.” In his “Fundamentals of Catholic Dogma,” Ludwig Ott observes, “Modern Christian philosophy generally holds that the creation and infusion of the spiritual soul coincides with the moment of conception.” Despite their clarity, however, these statements do not suggest a time for the creation and infusion of souls in twins, and, when asked, most modern moral theologians will admit they cannot say.
One reason, among others, is a lack of empirical biological data to demonstrate the exact moment the physical process of twinning begins. This is not to say the Church believes God — ever — abandons a human embryo to languish without a soul. On the contrary, the Catechism of the Catholic Church teaches, “It is because of its spiritual soul that the body made of matter becomes a living, human body” (No. 365). The question is not “if,” but “when.”
While the Church may not be able to answer the question precisely, its moral principles provide sound guidelines for approaching it. One states that in matters concerning human life, we must always assume life begins sooner rather than later.