ontological argument 
Anselm’s most famous work was a book called the Proslogion in which he outlines his Ontological argument in the form of a prayer spoken directly to God. As a firm believer in God, Anselm wished to prove God’s existence and confirm his strong faith by using logic and reason. The Ontological argument is a priori and is based on deductive reasoning because it seeks to prove the existence of God from the understanding of the attributes of the God of classical theism.
Chapter Two of the Proslogion introduces Anselm’s argument. The first part of the argument focuses on the definition of God. Anselm defines God as ‘that than which nothing greater can be conceived’. By this, he meant that God is the greatest being that can be thought of and is a being that cannot be improved upon. His aim is to refute the fool who says in his heart that there is no God. This fool has two important features: He understands the claim God exists and he does not believe that God exists. Anselm’s goal is to show that this combination is unstable. Anyone who understands what it means to say that God exists can be led to see that God does exist.
The fool understands the definition of God but denies that God exists. Anselm explains a distinction. It is one thing for an object to exist in the understanding and another for one to understand it to exist. Anselm’s point is that in general there is a difference between saying that something exists in the understanding and saying that one understands or believes it to exist. Trolls exist in the understanding; but one does not understand them to exist.
Anselm then applies the distinction he draws to the case of God. The fool understands claims about God. So God – a being than which nothing greater can be conceived – exists in his understanding. Anselm’s aim is to demonstrate that God cannot possibly exist in the understanding alone. This is because it is greater for a thing to exist in reality than for it to exist in the understanding alone.
Chapter Two of the Proslogion introduces Anselm’s argument. The first part of the argument focuses on the definition of God. Anselm defines God as ‘that than which nothing greater can be conceived’. By this, he meant that God is the greatest being that can be thought of and is a being that cannot be improved upon. His aim is to refute the fool who says in his heart that there is no God. This fool has two important features: He understands the claim God exists and he does not believe that God exists. Anselm’s goal is to show that this combination is unstable. Anyone who understands what it means to say that God exists can be led to see that God does exist.
The fool understands the definition of God but denies that God exists. Anselm explains a distinction. It is one thing for an object to exist in the understanding and another for one to understand it to exist. Anselm’s point is that in general there is a difference between saying that something exists in the understanding and saying that one understands or believes it to exist. Trolls exist in the understanding; but one does not understand them to exist.
Anselm then applies the distinction he draws to the case of God. The fool understands claims about God. So God – a being than which nothing greater can be conceived – exists in his understanding. Anselm’s aim is to demonstrate that God cannot possibly exist in the understanding alone. This is because it is greater for a thing to exist in reality than for it to exist in the understanding alone.