God this argument supports, and later thinkers such as from thinkers, both contemporary (like Gaunilo), in the centuries after (Aquinas), and in the modern era (Kant). In response to these criticisms, the ontological argument has changed, especially by Anscombe, and these subtle reforms have made it a watertight argument that means that if one has a true understanding of what God is and entails, one cannot without making a logical contradiction deny His existence.
Unlike many arguments for the existence of God which use empirical methods to prove His existence, the ontological argument proudly refrains from such a posteriori arguments based on empirical evidence, and instead aims to prove God’s existence from the definition alone. At first this seems strange, as this is at odds with our experience of truths and facts in our lives. In any normal circumstances, existential claims don’t follow from conceptual claims. For example, if I want to prove that bachelors, unicorns or viruses exist, one cannot simply reflect on the concepts themselves. If I want to prove the existence of such things, I must go out into the world and conduct some sort of empirical investigation, and likewise if I want to prove that such things do not exit. Thus in general, positive and negative existential claims can be established only by empirical methods.
However, some things within our human fathoming can be proved by definitions alone. To take another example, mathematical and geometrical concepts can exist by their precepts alone, and their properties can be determined by this. It is impossible, for example, to imagine a square circle or a triangle with four sides, so indeed it is impossible to envisage some things which can come about from their definition alone. Although some arguments for the existence of God are based on empirical information, such as the Teleological Argument (a.k.a. the Design Argument), the concept of God as a highest possible being does not require such evidence as a proof.
As it was originally formulated, the ontological argument rests on the following premises:
Premise 1: It is a conceptual truth (or, so to speak, true by definition) that God is a being than which none greater can be imagined (that is, the greatest possible being that can be imagined)
