Serge asked:

Could you kindly give an account of the ontological argument, which is meant to be one of the
attempts to prove God's existence?

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There are various forms of the ontological argument, the most famous ones being by St Anselm and
Rene Descartes. A slightly tongue-in-cheek summary of Anselm's version is to be found in Bluff your
way in Philosophy (Jim Hankinson, Ravette Books):

"It goes like this: think of something greater than which nothing can exist; but existence itself is a
property that makes something better. So if this greatest thing (i.e. God) doesn't exist, there would be
a yet greater thing imaginable, namely an existent God, having all the same properties as the other
one with the added bonus of existence. But we can conceive this. So God must exist."

Katharine Hunt