When Saul Kripke bifurcated necessity from the a priori in Naming and Necessity he also accidentally brought down the Ontological Argument for the existence of God.
Kripke’s groundbreaking research in analytic philosophy exposing that not all necessary truths are a priori truths led to the collapse of St. Anselm’s argument but Anselm’s argument turns on an assumption that something that is a priori true (i.e. "The quality of existence is contained in the concept of that which no greater can be conceived") must necessarily be true.
St. Anselm's ontological argument uses a logical sleight of hand that banks on the reader to conflate necessary truths as though they are a priori true as well. This just isn't the case.
There are a priori contingent truths, and a posteriori necessary truths and vice versa (see Kripke Naming and Necessity).
If you are unconvinced of the existence of a priori contingent truths read this.
Therefore the hidden premise in St. Anselm's argument that we can move from something that is a priori "That which no greater thing can be conceived also contains existence as a property as existence is greater than non existence" to concluding that this a priori fact must also be a necessary fact about the universe is false.
This area is an interesting area of philosophical research as it exposes the limits of pure cognition. Some facts can not be brought into existence by cognition alone.
Kripke shows us necessary truths aren't always true a priori. St. Anslem shows us a fallacy that occurs when we assume the set of the a priori truths and the set of all necessary truths are equal sets. Call it the necessirori fallacy. The "perfect island" rebuttal displays the reductio of St. Anselm's Ontological Argument.
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