Assess the Ontological Argument
Ontological arguments for God's existence are said to be deductive i.e. if we accept the premise, then the conclusion follows as a necessary truth (cannot be denied without contradiction). Ontological arguments are also founded upon purely a priori knowledge of God, as opposed to the four other types of argument which all use inductive reasoning and a posteriori knowledge. Ontological arguments are therefore favoured by rationalist theologians e.g. Rene Descartes (1596-1650) rather than more empiricist philosophers such as Thomas Aquinas, (1225-1274) whose arguments are all based on a posteriori knowledge and use inductive reasoning.
St. Anselm (1033-1109) put forward the first ontological argument in his book Proslogian, but it was only later named the ontological argument by Emmanuel Kant (1724-1804). This first version is as follows:
P1. God exists in the understanding as "that than which nothing greater can be conceived."
P2. It is greater to exist in reality as well as in the understanding.
C. Therefore God must exist in reality.
The monk Gaunilo however disagreed that we can simply imagine something into existence. He used the counter-example of an island to illustrate this:
P1. We can imagine the greatest conceivable island.
P2. It is greater to exist in reality as well as in the understanding.
C. Therefore the island must also exist in reality.
He also claimed that the first premise is not a necessary statement, but is rather contingent and requires a posteriori knowledge in order to be justified. As Aquinas put it: "Because we do not know the essence of God, the proposition ['God exists'] is not self evident to us."
Anselm's response to Gaunilo was based on an extension of his definition of God's necessity. He argued that because God is the greatest conceivable being, His existence is therefore necessary i.e. cannot be denied without contradiction. An island however can always be improved (e.g. even more beautiful women in bikinis) and so it's existence is contingent i.e. possible to deny without contradiction. "God cannot be conceived not to exist...That which can be conceived not to exist is not God."
- Proslogian, Ch 3.
