Final answer:
Anselm's ontological argument seeks to establish the necessity of God's existence through reasoning alone, defining God as 'a being than which nothing greater can be conceived and using a reduction ad absurdum to prove that assuming God's non-existence leads to a contradiction.
Step-by-step explanation:
One purpose of an ontological argument for God's existence by Anselm is to establish the necessity of God's existence through an a priori explanation of the concept of existence or necessary being, without relying on empirical data. Anselm defines God as "a being than which nothing greater can be conceived." His argument is a reductio ad absurdum that suggests assuming the non-existence of God leads to a logical impossibility. Anselm begins by defining the central term - God - and sets the foundation for an argument built on reason alone. If God is understood as the greatest conceivable being, then even in concept, God must exist, because the idea of a God that exists is greater than one that does not. The contradiction in the assumption of God's non-existence leads to the conclusion that God must indeed exist.