Why the Ontological Argument Fails?

In the end, the Ontological Argument fails as a proof for the existence of God when careful attention is paid to the cognitive terms that it employs. When the terms are disambiguated, either nothing philosophically interesting follows or nothing follows at all.

Why did Kant reject the ontological argument?

Because we cannot experience God through experience, Kant argues that it is impossible to know how we would verify God's existence. This is in contrast to material concepts, which can be verified by means of the senses.

What was Gaunilo's main problem with the ontological argument?

In his work In Behalf of the Fool, Gaunilo contends that St Anselm's ontological argument fails because logic of the same kind would force one to conclude many things exist which certainly do not. An empiricist, Gaunilo thought that the human intellect is only able to comprehend information provided by the senses.