“For what can be known about God is plain to them, because God has shown it to them. For his invisible attributes, namely, his eternal power and divine nature, have been clearly perceived, ever since the creation of the world, in the things that have been made. So they are without excuse” (Romans 1:19-20 ESV).
Before one embarks on theological enterprises, one must presuppose that a God exists, or else prove to oneself that His existence is real. In the 13th century, the great Christian thinker Thomas Aquinas proposed five proofs in his Summa Theologica, a classic and influential philosophical work. These proofs, called quinque viae or “five ways”, compose the classical arguments for God’s existence.
The first argument, the argument from motion, also called the unmoved movement argument, comes from the physics of inertia, whereby an object that is in a certain state will stay in that state until impressed by an external force. Aquinas adapted this argument from unmoved mover idea that Aristotle discussed in Metaphysics. One sees that what moves is moved by something. A moves because B moved A and B moves because C moved B, on to infinite, unless one assumes that a First Mover exists who can cause movement without needing to be moved itself.
The second argument, the cosmological argument, is similar to the argument from motion. Aquinas again drew inspiration from Aristotle. The cosmological argument concerns causation. That which exists exists because some other existence caused it, and the existence that caused the first to exist exists because yet another existence caused it. Imagine the cosmological argument as dominoes falling into each other. Who caused the dominoes to start falling?
The third argument, the ontological argument, came from Saint Anselm of Canterbury. Ontology has to do with being, so the ontological argument is based on the necessity of God existing. Anselm reasoned that God exists because one can conceive of Him. By definition God is the most supreme being. If He exists only as a mental construct, then one could conceive of some being greater than Him. As it is, one have already defined Him as the most supreme being, so this is contradictory. Therefore, God exists out of necessitation.
The fourth argument, the transcendental argument, posits that God must exist for reason, logic, and morals to exist; He is a pre-condition, because in affirming these one assumes their pre-existence. If God does not exist, then thinkers can prove nothing because they will have no basis for their reasoning. Reason/logic/morals would be a man-made conventions, which means that they are not universal, as culture and time could change them.
The fifth argument, the teleological argument, also called the argument from design or intelligent design argument, evaluates the perceived deliberate design or purpose of the world for proof of God. The world contains such intricacy and order that it could not have come together by chance. In the 19th century William Paley explained the teleological argument in terms of a watch. One does not suppose that a watch exists by its own volition; rather, one knows that a watchmaker exists who made the watch. Likewise, creation, which boasts much more complexity than a mere watch, must have had some greater being who made it.
From this a posteriori reasoning Aquinas concluded that God existed. He identified God as the first mover, the uncaused cause, the necessary reality, the requirement for logic, and the intelligent designer that met the demands of his five ways.