The ontological argument was proposed by Anselm of Canterbury in the second chapter of his Proslogion. Although he did not propose an ontological system, he was very much concerned with the nature of being.
He distinguished necessary beings (those that must exist) from contingent beings (those that may exist, but whose existence is not necessary).
Anselm of Canterbury
- 1. If I am thinking of the Greatest Being Thinkable, then I can think of no being greater
- 1a. If it is false that I can think of no being greater, it is false I am thinking of the Greatest Being Thinkable
- 2. Being is greater than not being
- 3. If the being I am thinking of does not exist, then it is false that I can think of no being greater.
- 4. If the being I am thinking of does not exist, then it is false that I am thinking of the Greatest Being Thinkable
Conclusion: If I am thinking of the Greatest Being Thinkable, then I am thinking of a being that exists
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