Proving the Existence of God Anselm vs. Aquinas

Anselm argumentation is known as the a priori proof for the existence of God or the ontological argument. In his work "Proslogion," chapters 2 and 3, "That God Really Exists" and "That God Cannot be Thought Not to Exist," are the key to the entire Anselmian philosophy on the existence of God. He uses a reduction ad absurdum argumentation to prove God's existence.

Indeed, he starts from the concept of "a being than which no greater can be conceived"

In his work, he says, referring to God, that we believe He is "something greater than which cannot be thought"

Further more, this something greater than can be conceived may exist in someone's thought, but, on the other hand, it cannot exist only in one's thought, but also in reality ("certainly that greater than which cannot be understood cannot exist only in thought, for if it exists only in thought it could also be thought of as existing in reality as well, which is greater"

). The final contradiction is that something greater can actually be conceived which contradicts our initial supposition that nothing greater could be conceived. This proves that God exists not only in one's thought, but also in reality.

The a posteriori argument that Aquinas aims to present is somewhat different in the sense that God's existence is attempted to be proven by the existence of a causality between some of the consequences we see in real life and the Cause that has produced them.

We have, for example, the argument of from motion. We notice in the nature that objects move (in the 13th century, when Aquinas wrote, it was most likely that they moved from natural sources rather than artificially). As something is moved, it is physically necessary for something to move it from the outside. At the same time, there must be an initial first mover that is not moved and which we may identify with the Supreme Force or God: "it is necessary to arrive at a first mover, put in motion by no other; and this everyone understands to be God"

Similarly, the argument of the First Cause relies on the existence of an initial will from which all subsequent causes derived. Additionally, there must be an initial principle that provides for its own necessity and a primary factor that is the initial creator.