Answer: St. Thomas Aquinas (c1225-1274) is arguably the most important Catholic theologian in history. In his major work Summa Theologica, widely considered as the highest achievement of medieval systematic theology, Aquinas presented his five proofs of God's existence known as the Quinque Viae (Latin for "Five Ways"). [1] We will be presenting all the arguments in more detail a little later, at present we will give a brief rundown of all five arguments.
* Thomas' first way involves the evidence of motion. The fact, to Thomas, that every moving thing needs a mover shows that God, the Unmoved Mover, exists.
* The second way involves the notion of efficient cause. For the series of causes and effects, that we see in the world, to make sense it must have a beginning. God, the First Cause, therefore exists.
* The third way notes that every existing thing does not owe its existence to itself. However, if all things are contingent, there could not have been anything as at one time all these could be non-existent. To account for all existence, there must be a Necessary Being, God.
* The fourth way shows that there exist gradations in things, for example more noble and less noble, more true or less true. The existence of such gradations implies the existence of an Absolute Being as a datum for all these relative gradation.
* The fifth way argues that the behavior of things in the world implies a Grand Designer or architect, God. [2]
Thus Aquinas' five ways defined God as the Unmoved Mover, the First Cause, the Necessary Being, the Absolute Being and the Grand Designer.
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