Hi inocente,
This still doesn’t make any sense.
An entity exists necessarily if it cannot fail to exist. Where is the contradiction in this definition? It is not apparent as in the cases round squares, four-sided triangles and such.
But, we are told, we can translate our definition of necessary existence into: An entity exists necessarily if it exists in every possible world. What is a possible world? It is any conceivable way the actual world could have been. In conceiving a possible world, the only rule is that when we change any element of the actual world we don’t create nonsense. If it false that unicorns exist in the actual world, there is nothing illogical if we suppose it is true. And so it is with each and every element of the actual world. the T/F or exists/doesn’t exist switches can be flipped as to any element.
Now given these simple rules we can tell right away that it is impossible for anything to exist in every possible world, because I can always conceive of a given entity as not existing. No need to engage in Vera’s possible world reduction thought experiment. Take any X and flip the switch to “doesn’t exist” and I’ve done it.
But wait a minute, why is it true that we can always conceive of any given entity as not existing? Just because.
You’re doing the same as Vera - by flipping the switch you’ve defined X not to exist. But that scheme is just what’s called a truth table, the purpose of which is to lay out every combination of possibilities, including X=true and X=false. So of course it includes at least one line where X doesn’t exist, and includes at least one line where it does.
Unfortunately that not how modal logic and possible worlds frameworks work. Here’s Plantinga’s modal logic ontological argument, as he wrote it:
(25) It is possible that there be a being that has maximal greatness.
(26) So there is a possible being that in some world W has maximal greatness.
(27) A being has maximal greatness in a given world only if it has maximal excellence in every world.
(28) A being has maximal excellence in a given world only if it has omniscience, omnipotence, and moral perfection in that world.
,] it follows that there actually exists a being that is omnipotent, omniscient, and morally perfect; this being, furthermore, exists and has these qualities in every other world as well. For (26), which follows from (25), tells us that there is a possible world W’, let’s say, in which there exists a being with maximal greatness. That is, had W’ been actual, there would have been a being with maximal greatness. But then according to (27) this being has maximal excellence in every world. What this means, according to (28), is that in W’ this being has omniscience, omnipotence, and moral perfection in every world.
Note Plantinga defines what he means by greatness.