As a theologian and philosopher, Saint Anselm strove to prove the existence of God in reality. The bulk of his argument is found in Chapter II of Proslogium.
Anselm begins by defining God as ‘a being than which nothing greater can be conceived’;. He continues by stating that ‘even a fool’; has the capacity to understand this definition of God and that whatever is understood exists in the understanding. Anselm now draws his first intermediate inference based on these initial premises; God must exist in the understanding, and is therefore a possible being. Aneselm next draws on the premise that if something exists in reality, it is greater than if it exists in the understanding alone. At this point in his argument Anselm switches tactics and supposes that God exists only in the understanding. Based on the former premise this would mean that is possible that God (had He existed in reality) might have been greater than He is (existing only in the understanding). Based on that supposition, God is not the being than which none greater is possible. If Anselm’s initial definition of God is substituted into the previous inference, it becomes a contradictory statement: the being than which none greater is possible is not the being than which none greater is possible. Therefore Anselm supposition that God exists only in the understanding is false. By proving this to be invalid Anselm has, in effect, proven that God must exist in the understanding and reality. This final conclusion, that God must exist in reality, is the goal of Anselm’s argument.
Chapters III, and IV of Proslogium support Anselm argument by explaining in depth the definition of God. He asserts that nothing greater can be conceived that is not God and that if a mind could conceive of a being better than God, that creature would rise above God. In Chapter V Anselm proceeds to deduce God’s nature from the same basic definition of Him as something greater than which cannot be thought. He arrives as all the standard attributes: ‘just, truthful, blessed, and whatever it is better to be than not to be’;.
Many objections have been raised as to the validity and soundness of Anselm’s ontological argument. One of the most compelling and most famous objections was present by Immanuel Kant in the eighteenth century. Kant found fault with Anselm’s premise that if something exists in reality, it is greater than if it exists in the understanding alone. According to this objection, ‘existence’; is not a characteristic or property. Therefore a thing’s greatness would depend solely on what properties it has; whether or not something exists in reality or only in the understanding does not affect its greatness in any way. For example, consider an imaginary one hundred dollar bill with all the same properties as a one hundred dollar bill that does exist in reality. The only difference between the real bill and the imaginary one is that the former exists both in reality and in the understanding while the latter exists only in the understanding. But this difference is not a difference in any set of characteristics or properties that the bills have. Existence in reality is not like being green or being made of paper; it is not a characteristic or property at all. By finding this single premise to be unsound, Anselm’s entire argument is refuted.
Anselm might respond to Kant’s objection by stating that the possible beings that exist only in the understanding must be contingent things. These contingent things might either exist or fail to exist. Necessary things are greater than contingent things because they cannot fail to exist. Therefore Anselm’s premise should be understood as saying that if something exists only in the understanding, and is a possible being, it is a contingent being. But being a contingent being, it could have been greater than it was as a necessary being. By explaining the premise in the way listed above, Kant’s objection is no longer relevant.