St. Thomas Aquinas (1225–1274) was a renowned Doctor of the Church, a title given by the Roman Catholic Church to saints who made important contributions to theology. Aquinas studied the works of the Greek philosopher Aristotle in great depth. His “first cause” (prima causa) argument on the existence of God is modeled on an idea set forth by Aristotle many centuries earlier.
In book 8 of Physics and book 12 of Metaphysics, Aristotle discusses his concept of the “unmoved mover,” an “immortal, unchanging being ultimately responsible for all wholeness and orderliness in the universe.” This being is never acted upon by other forces, but sets into motion all the chain reactions of every kind that take place in the universe.
In his Summa Theologica, St. Thomas adapts Aristotle’s concept of “the unmoved mover” and applies it to Christian theology, calling this idea “first cause.” God is the first cause of everything, but exists outside of the realms and limitations of space and time as we know them. God is the initiator of all kinds of chain reactions and connecting events—a kind of supreme catalyst.
Your question on whether this argument actually proves the existence of a traditional God of theism depends on whether one looks at it from a logical or a theoretical point of view. Some questions cannot readily or appropriately be put to the test of deductive reasoning or logic, but this does not necessarily negate their validity or importance.
German philosopher Immanuel Kant rejected the argument of “first cause” on the basis that logically, one cannot trace back known facts to a cause that belongs to an entirely different category—that of metaphysics or transcendence.
The idea of “the unmoved mover” or “first cause” provides a possible explanation for a great and eternal mystery—even if the argument itself does not follow the rules of deductive reasoning.