In the coming road to establishing the conclusion of the Ontological Argument, we have probably all heard the all too common objection that “existence is not a predicate.” Since, as Anselm argues in his Proslogion (“A Discourse”), if something only exists in the understanding and it is possible for it to exist in reality, then that thing has the possibility of being greater than if it merely existed in the understanding alone. Thence, through the chain of reasoning, a being by which none greater can be conceived (i.e., “God” – GPB hereafter) is only great as such if it were to exist both in the understanding and in reality. Therefore, God (so defined) exists.
Of course, this is on the basis of Anselm’s assumption that GPB does not exist. He thence further argues that from this conceptual understanding of such a being we can stipulate the actual existence of the thing so defined. However, one tradition of common critics suggest that “existence is not a predicate”, which suggests that we cannot apply existence to the mere concept of a thing and say that it exists in reality. Consider for example when I say “Steven is tall.” This is a proposition denoting some feature of a given person named Steven who is tall. However, in applying that predicate (tall) to Steven, we are presupposing that Steven actually exists.
This line of thinking has gone on to say that if existence were a predicate, then certain existential propositions are either tautologies or are self-contradictory. Considering the difference between (1) affirmative existential propositions (x does exist) and (2) negative existential propositions (x does not exist), x is still a given thing that we presuppose to exist. Thus, affirming x’s existence would just be redundant, and denying x’s existence would be self-contradictory – since, in predicating x we are presupposing that x exists: why would we say then that x does not exist? This has been argued for instance by A.J. Ayer in his book, Language, Truth, and Logic (1936). As he writes,
Here again the fact that there is a superficial grammatical resemblance between the English sentences “Dogs are faithful” and “Unicorns are fictitious,” and between the corresponding sentences in other languages, creates the assumption that they are of the same logical type. Dogs must exist in order to have the property of being faithful, and so it is held that unless unicorns in some way existed they could not have the property of being fictitious. But, as it is plainly self-contradictory to say that fictitious objects exist, the device is adopted of saying that they are real in some non-empirical sense – that they have a mode of real being which is different from the mode of being of existent things. 
This is obviously problematic since this objection itself presupposes that concepts apply to the real world. In other words, discussions about a given existential proposition – whether affirmative or negative – regarding the concept of the proposition, must apply to the actual existence of the given thing that the proposition is about. Therefore, saying that “Steven loves dogs” is a proposition that denotes a given person Steven who loves dogs. The predicate of which (“…loves dogs”), is only subscribed to the character stated in the proposition. The proposition by no means is presupposing that Steven actually does exist or does not exist.
William Rowe (1974) is not satisfied with this objection, dismissing it on the same grounds from which we are discussing here. To finish our discussion:
The plain fact is that we can talk about and ascribe predicates to many things which do not exist and never did. Merlin, for example, no less than Houdini, was a magician, although Houdini existed but Merlin did not. If, as these examples suggest, the claim that whenever we ascribe a predicate to something we assert or presuppose that that the thing exists is a false claim, then we will need a better argument for the doctrine that existence is not a predicate. 
-  A.J. Ayer, Language, Truth and Logic (Dover Books: 1952) p. 43
-  Quoted from Reason and Responsibility, ed. Joel Feinberg and Russ Shafer-Landau (Thomson and Wadsworth: 2005) pp. 16-17