Therefore the idea of a non-existent greatest possible being is therefore a contradiction in terms – so God must exist as a necessary being; his non-existence is logically impossible.
In the 13th century Aquinas argued that the ontological argument does not work, as we cannot form a conception of God in our minds. This was challenged in the 17th century by the French philosopher and mathematician Rene Descartes, who reformulated the ontological argument in his work ‘Meditations’.
Descartes agreed with Plato that we are born with innate ideas; he argued that there are some concepts which are imprinted on our minds at birth and which are universally shared by all human beings.
Descartes thought that we understand such mathematical concepts as equality, cause, shape and number from birth, and he also believed that we are born with an understanding of what God is.
We understand God to be the supremely perfect being, with all the perfections as his attributes, such as omniscience, omnipotence and omnibenevolence.
The concept of a perfect being must therefore have originated from the perfect being itself. Furthermore, a perfect being must have all perfections, in other words it must be perfect in every respect and not lack anything.
Therefore a perfect being must exist in order to be perfect, otherwise it would lack something – existence.
Descartes explained his understanding of how the ontological argument works by using the analogy of a triangle, and also the analogy of a mountain. Descartes claimed that existence is part of the essence of God, just as three angles adding up to 180 degrees are part of the essence of a triangle, and a valley is part of the essence of a mountain.
Just as you cannot have a triangle without three angles or a mountain without a valley, so you cannot have a God who does not exist.
As Descartes expressed it, ‘Existence can no more be separated from the essence of God than can its having three angles equal to two right angles be separated from the essence of a triangle, or the idea of a mountain from the idea of a valley’.
Descartes recognised that analogies have their limitations. While we may not be able to think of a mountain without also thinking of a valley, this does not mean that the mountain-and-valley combination in our imaginations actually exists in the real world. But Descartes claimed that God was different, because his nature involves not angles or valleys but perfections, and for Descartes existence is a perfection. Because God possesses all the perfections, and existence is a perfection, God therefore must exist.
St Anselm is widely credited with inventing the ontological argument in his work ‘Proslogion’, written in 1078 when he was Archbishop of Canterbury. Anselm begins with a passage from the Bible – “The fool has said in his heart there is no God” (Psalm 53).
He then proceeds to show by a ‘reductio ad absurdum’ that the fool really is a fool, as he believes in something that is self-contradictory.
Anselm begins by looking at all the traditional attributes of God and reaching the conclusion that God is ‘that than which nothing greater can be conceived’.
However, to say that such a being exists only in the mind is an absurdity, as we can conceive of something greater than this, namely the same being existing in reality. For it is greater to be a being that actually possesses qualities such as omnipotence and omniscience, than a being that is restricted to the mind and cannot possess any qualities at all.
In conclusion, Anselm states that existence may be ‘in re’ (in reality) or ‘in intellectu’ (in the mind) and the former is undeniably greater. Since God is that which nothing greater can be conceived, he must by necessity exist ‘in re’, otherwise God is not the greatest being we can conceive, and this is a self-contradiction.
Anselm wrote a second version of his ontological argument, in which he focused on the distinction between necessary existence and contingent existence to show the uniqueness and distinctiveness of the nature of God. Anselm wrote a second version of his ontological argument, in which he focused on the distinction between necessary existence and contingent existence to show the uniqueness and distinctiveness of the nature of God.