Why ontological arguments are illogical

[Oldandeasilyconfused]

Logic guarantees truth if and only if the premise is true. An argument may be perfectly logical and the inference valid but untrue,or at least unable to be claimed to be true. The ontological argument is logically valid,but the inference may not be claimed to be true.

I found the Wiki article cited below very helpful,especially the bits on Rene Descartes,and "the Muslim Plato" The mulla Sadra (Sadra al-Din Muhammud al-Shirazi)

An ontological argument for the existence of God (or simply ontological argument) is any one of a category of arguments for the existence of God. The exact criteria for the classification of ontological arguments are not widely agreed, but the arguments typically start with the definition of God and conclude with his necessary existence, using mostly or only a priori reasoning and little reference to empirical observation.

http://en.wikipedia.org/wiki/Ontological_argument