Final answer:
Anselm distinguished between necessary beings, which must exist, and contingent beings, which may exist but are not necessary. He argued that if God is the greatest conceivable being, then God must exist necessarily, as existence is a perfection inherent to the concept of God.
Step-by-step explanation:
Anselm's argument, which is part of his ontological arguments, makes a distinction between two types of beings: the necessary being and the contingent being. A necessary being is one that must exist and cannot not exist, which is central to Anselm's argument for the existence of God. God, in Anselm's view, is a necessary being and the greatest conceivable being, meaning that the concept of God necessarily entails existence. On the other hand, contingent beings are those that can exist but are not required to; their existence depends on something else.
Anselm argued for the reality of this distinction by claiming that the idea of God implies necessary existence — if you can conceive of the greatest possible being, you must also conceive that this being exists necessarily, for a being that does not exist cannot be the greatest. To suggest that a necessary being like God might not exist would be a contradiction, as existence is a perfection attributed to the concept of the greatest conceivable being. Therefore, according to Anselm, God's existence is logically obligatory.