Descartes (yeah, the 'I think therefore I am' blokey) formulated another version of this argument.
He claimed that there were two types of statement: analytical and synthetic.
A synthetic statement is one which may or may not be true, such as 'bears are brown'. One must go outside the statement to look for evidence for or against it.
An analytical statement is one which is a priori true, such as 'triangles have three corners'. It is impossible to seperate the subject (triangles) from the predicate (having three corners).
In other words, an analytical statement states a defining characteristic of the subject, and so needs no external evidence to prove that it is true.
Descartes maintained that 'God exists' is an analytical statement: existence is one of the defining characteristics of God and so needs no outside evidence.