|(Warren Dunes State Park, Michigan)|
Graham Oppy presents Alvin Plantinga's modal version of the ontological argument as follows.
An entity is "maximally great" iff (if and only if) it necessarily exists and possesses "maximal excellence" (i.e., is omnipotent, omniscient, and morally perfect).
Keeping this in mind, note that a maximally great entity cannot be a contingent thing. As regards contingent things, it is possible that a certain contingent thing exists. E.g., it is possible that a unicorn exists (logically possible).
Therefore, regarding a maximally great thing:
1) Either it is not possible that a maximally great entity exists or it is necessary that a maximally great entity exists.
2) It is (logically) possible that a maximally great entity exists.
3) Therefore a maximally great entity exists. (That is, an entity that is omniscient, omnipotent, morally perfect, and possesses these attributes (i.e. is "maximally excellent") in every possible world.)
Oppy frames it this way.
1. There is a possible world in which there is an entity which possesses maximal greatness.
2. (Hence) There is an entity which possesses maximal greatness.
Oppy writes: "Under suitable assumptions about the nature of accessibility relations between possible worlds, this argument is valid: from it is possible that it is necessary that p, one can infer that it is necessary that p."
1) it is possible that it is necessary that p.
2) Either p cannot possibly exist or p necessarily exists.
3) Therefore (from P1 & P2, using disjunctive syllogism) p necessarily exists.
Oppy's essay on the Ontological Argument in Stanford Encyclopedia of Philosophy is a good one. He goes on to offer criticisms of Plantinga's version. And gives Plantinga's further reflections on the status of the argument.