In the Third of his celebrated Five Ways in Summa Theologiae Ia, q. 2, a. 3, St. Thomas Aquinas argues for the existence of God from contingency and necessity noting that the world contains possible beings which are able not to be since, being generated and corrupted, they at some time do not exist. He claims to show that there must be some necessary being since it is impossible that all things are possible beings. Scholars have long found this part (...) of the Third Way problematic, since it appears that Aquinas is committing a logical fallacy. He seems to be arguing that all members of a group have one common attribute since they each have their own particular instance of that attribute, as one might argue that since everybody has a birthday, i.e., one day on which each was born, therefore, there is one (and the same) day on which everyone was born. This paper will show that the accusation that Aquinas commits the alleged Birthday Fallacy would mean he was arguing for a temporal beginning for all material things, and so believed that such claim was philosophically demonstrable. Aquinas, however, argued against our ability to naturally know physical creation had a beginning in time; such knowledge requires God’s revelation of it. Instead Aquinas in the Third Way is allowing for the possibility that the physical universe could be temporally infinite, as Aristotle argued, but that this possibility would still support Aquinas’ contention that some necessary being must exist. (shrink)