Anselm’s “Ontological Argument”
The general idea of the ontological argument is based on the notion that the concept of God as the greatest being implies that God exists—if not, there could be something greater, namely an existent greatest being—but this being would be God.
The structure of the Ontological Argument can be outlined as follows (The argument is based on Anselm 's Proslogion 2):
1. We conceive of God as a being than which no greater can be conceived.
2. This being than which no greater can be conceived either exists in the mind alone or both in the mind and in reality.
3. Assume that this being than which no greater can be conceived exists in the mind alone.
a. Existing both in the mind and in reality is greater than existing solely in the mind.
b. This being, existing in the mind alone, can also be conceived to exist in reality.
c. This being existing in the mind alone is not therefore the being than which no greater can be conceived. (See statement 1 above.)
4. Therefore, this being than which no greater can be conceived exists in reality as well as exists in the mind.

Thomas Aquinas, “The Five Ways”
1. Part I. The Argument from Motion. (Thomas argues that since everything that moves is moved by another, there must thereby exist an Unmoved Mover.)
a. Evident to our senses in motion—the movement from actuality to potentiality. Things are acted on. (Again, note that the argument proceeds from empirical evidence; hence it is an à posteriori or an inductive