In a recent article of mine entitled “Why I Reject Anselm’s Ontological Proof” (2013), I argued that the proposition “God exists” being an existential proposition by no means entails that the person this proposition denotes is by any means logically necessary. In line with the reasoning of Mortimer Adler (1980), I would certainly suggest that “When we have direct perceptual acquaintance with a particular individual, the existence of that individual is evident to us, but the proposition which asserts that the individual exists is not self-evident, nor is its truth necessary” .
Hence, I would reject Anselm’s argument on the grounds that existential propositions – propositions that suggest x exists – are not necessary, and thus do not entail the actual existence of a thing from the mere conceptual framework of that thing. However, is that really an issue once we consider Plantinga’s argument? Alvin Plantinga (1977) himself admits that at “first sight Anselm’s argument is remarkably unconvincing if not downright irritating; it looks too much like a parlor puzzle or word magic” . The argument moreover has not played much of a role in coercing religious belief. Yet, philosophers over the past century have raised considerable discussions on Anselm’s argument.
Plantinga suggests that the reason might be because of the ontological/metaphysical problems of philosophy inherent within the argument. As Plantinga asks, “Is existence a property? Are existential propositions – propositions of the form x exists – ever necessarily true? Are existential propositions about what they seem to be about? [ … ] These issues and hundred others arise in connection with Anselm’s argument” .
As a frame of reference, consider the restated argument as follows :
- (1) It is possible that there be a being that has maximal greatness.
- (2) So there is a possible being that in some world W has maximal greatness.
- (3) A Being has maximal greatness in a given world only if it has maximal excellence in every world.
- (4) A being has maximal excellence in a given world only if it has omniscience, omnipotence, and moral perfection.
This argument avoids the previous dilemmas of necessary existence being a perfection, since, a being cannot be omnipotent (or all-knowing, wholly good, etc.) in a given world unless it exists in that world. With (1), (3), and (4) taken collectively, you receive the conclusion that such a being who is omniscient, omnipotent and morally perfect actually exists. I think the argument becomes interesting once we consider (3) and (4) as consequences of a definition; Plantinga explains:
Accordingly these premises, (1), (3), and (4), entail that God, so though of, exists. Indeed, if we regard (3) and (4) as consequences of a definition – a definition of maximal greatness – then the only premise of the argument is (1). 
Therefore, the defender of the ontological argument is only left to deal with premise (1): that it is possible that such a being so defined exists. However, premise (2) which follows from (1), speaks of a possible having such characteristics of maximal greatness. Hence, we are left to address the concerns of what exactly it means to be a possible being. As Plantinga suggests, “the version of the ontological argument we’ve been considering seems to make sense only on the assumption that there are such things” . To fix this dilemma of assumption, Plantinga thence invokes instead the property of maximal greatness, or being maximally great. The premise corresponding to (1) then states that maximal greatness is possibly instantiated in a given being; or, that
- (5) There is a possible world in which maximal greatness is instantiated. And the analogues of (3) and (4) spell out what is involved in maximal greatness:
- (6) Necessarily, a being is maximally great only if it has maximal excellence in every world
- (7) Necessarily, a being has maximal excellence in every world only if it has omniscience, omnipotence, and moral perfection in every world.
Hence, with the conjunction of (6) and (7) we no longer have the problem of dealing with possible but nonexistent beings. As Plantinga finishes: “if (5) is true, then there is a possible world W such that if it had been actual, then there would have existed a being that was omnipotent, omniscient, and morally perfect; this being, furthermore, would have had these qualities in every possible world. So it follows that if W had been actual, it would have been impossible that there be no such being” . Hence, if W had been actual,
- (9) There is no omnipotent, omniscient, and morally perfect being
would have been an impossible proposition. Furthermore, impossible propositions are not only impossible in one world, but in every world. Therefore, there actually exists a being that is omnipotent, omniscient, and morally perfect, and who exists in every possible world.
-  Mortimer Adler, How To Think About God (Touchstone: 1980) p. 104
-  quoted from The Mystery of Existence, ed. John Leslie and Robert Lawrence Kuhn (Wiley-Blackwell: 2013) p. 115
-  Ibid.
-  The original passage can be found in part 2, section c, of God, Freedom, and Evil (William B. Eerdmans: 1977) pp. 85, 108-112
-  Leslie and Kuhn (2013), p. 116
-  Ibid., p. 117
-  Ibid.