The ontological Argument

Though Anselm's argument may initially seem quite convincing, there are numerous criticisms that refute his argument's structure, premises, and suppositions.

Gaunilo of Marmoutiers attacks the structure of Anselm's argument. Liber Pro Insipiente (on behalf of the fool) he suggests one may deduce the existence of any perfect thing from the mere concept of the thing itself using Anselm's argument structure, and that alternatively it was impossible to visualize any perfect thing without such a perfect thing actually existing! Such absurd consequences led him to believe the argument itself was flawed. Regrettably, Gaunilo's criticism speaks nothing as to where Anselm's specifically made a mistake, and cannot challenge the validity of the argument.

German philosopher Immanuel Kant (1724-1804) objects that Anselm's third premise, existence is a great-making property, is not a predicate. Kant claimed when we make a statement about a subject, we implicitly presuppose the existence of the subject, thus existential statements would be redundant (i.e. Tigers [which exist] exist.) One might object to Kant in saying we do not always presuppose existence when talking about mythical creatures (sirens, chimaeras).

Lastly, the "paradox of the stone" attempts to diminish Anselm's argument by challenging his assumptions from which he derives God's existence. Anselm claims the fool at least understands the concept of God, but is his concept of God not flawed and controversial in itself? One might ask, "Can God create a stone he cannot lift?"