Despite many attacks from Thomas Aquinas, David Hume, Immanuel Kant and Bertrand Russell, the ontological argument has continued to be popular and to present a strong case for providing grounds for providing the existence of God by use of reason alone. The argument fundamentally rests on the premise that there is a universe, but its existence is contingent (depending on something else to exist). Whatever provides the explanation for the universe’s existence cannot be contingent itself, but is necessary. It argues deductively, not inductively thus holding out the hope of a universal proof not dependent on empirical evidence about which we may be mistaken.
The form of the argument
The ontological argument is A Priori (concerned with being). It does not rely on the senses or the world around us for either its premises or conclusion. Instead, it moves in stages of logical argument to a conclusion which is self-evidently true or logically necessary.
The argument is analytic which means that as long as its premises are correct so is the conclusion, hence the argument reaches conclusions about the existence of God that are based on the definition of God used in the premises.
Origins
Anselm of Canterbury prayed for a single, short argument which would prove almost everything about God, including his nature and existence. For Anselm of Canterbury the existence of God held by him by the virtue of faith, was now also true by logical necessity, relying only on the analysis and meaning of the terms.  Anselm of Canterbury was trying to prove the existence of God by means of reductio ad absurdum - this method of reasoning aims to demonstrate the truth of something by reducing to absurdity the very opposite of what you are aiming to prove. In Anselm of Canterbury’s case, the opposite of his conclusion would be that God does not exist, which he aimed to show to be absurd by means of an argument demonstrating that the existence of God is logically necessary.
Context
- The definition of God as ‘that than which nothing greater can be conceived’ and the implications of this.
- Why the non-existence of God is logically impossible.
- Why ‘the fool’ believes that which is impossible to be true.
Defining God
Anselm of Canterbury’s argument is based on the very definition of the word God and its meaning. He makes an assumption which is crucial for the argument to work, which is that ‘God’ is effectively shorthand for ‘that than which nothing greater can be conceived’ or ‘the being than which nothing greater can be thought’.
God is greater than all other beings in the sense that he is supremely perfect - it must possess all perfections in order to be so described and when we speak of God we speak of such perfect being.
Anselm of Canterbury argues that if such a being does indeed possess all perfections then he must exist. This assumption is based on the idea that existence itself is a perfection. Anselm of Canterbury places existence into the same category as he would place goodness, he treats it as a defining characteristic.
Existence may be possessed or lacked, and that to possess existence is necessarily greater than to lack it. That which exists is the mind may hypothetically possess all other great-making qualities, but that which exists in reality is undeniably greater.
Now we believe that thou are a being than which none greater can be thought … clearly that than which a greater cannot be thought cannot exist in the understanding alone. (Anselm of Canterbury)
Without doubt, therefore, there exists, both in the understanding and in reality, something than which a greater cannot be thought (Anselm of Canterbury)
Such existence, Anselm of Canterbury maintains, is undeniably greater than existence in intellectu, and since God is that than which nothing greater can be conceived, God must possess the perfection of existence both in reality and in the mind.
The concept of necessary existence
God is of a being possessing necessary existence, it is de dicto necessary - by definition. Because the definition of God requires that he should exist, to deny his existence would be absurd.
The fool has said in his heart there is no God (Psalm 53)
Had the atheist grasped the real meaning of God as that than which nothing greater can be conceived, it would be impossible for him to deny his existence.
Even the fool, then, must be convinced that a being than which none greater can be thought exists at least in his understanding (Anselm of Canterbury)
Strengths
It holds out the hope of a proofÂ
It is a deductive argument. If valid, it will be proof for both believers and atheists.
Its starting point is valid for both believers and atheistÂ
The definition of God as ‘that than nothing greater can be conceived’ is accepted by the atheist, even if the atheist denies that such a being exists. The atheist must have an understanding of God in order to be able to reject belief in God.
It is an intellectually stimulating argument that continues to be studied and debated
It is reasonable to assume, therefore, that there are good reasons to consider the argument, in some way, to be sound.
Weaknesses
Not coherentÂ
How can God be omniscient? He cannot know human future choices.
Mutually InconsistentÂ
No being could be both omniscient and omnipotent, since an omnipotent being could make a creature who had a secret unknown to anyone but itself, while an omniscient being must know every secret.
Leading to a useless GodÂ
Even supposing we can make sense of the great-making properties and show them to be mutually consistent, won’t the concept of God that we arrive at be so distant from religious experience as to be useless?
Assumed by [[Anselm]] to be beyond criticismÂ
It cannot be assumed that this is the only logical way of defining God. For the process theologians, for example , a better definition of God is as ‘the fellow sufferer who understands’. Other believers may also be satisfied with a definition of God which permits him to be understood as less than supremely perfect.