This type of necessity is important to Rationalism because it deals with things that go beyond the type of logical certainty already discussed. Most attacks upon Rationalism have focused on this concept and it has been argued that there is no such thing as metaphysical necessity. An example would be the idea of God:

a) It is logically possible that God does not exist;
b) However, the idea of God presupposes a perfect being;
c) For a perfect being not to exist would be an imperfection;
d) Therefore God must necessarily exist.

This argument - called the Ontological Argument - assumes the existence of this third type of necessity (metaphysical) which sits midway between what can and cannot be otherwise.

This type of knowledge poses a question: can we argue from abstract principles to actual states of affairs? In other words, we might be able to describe a perfect island, but that doesn't mean that the island actually exists. Does it?

Discussion

Is it a convincing argument to say that such a perfect island must exist because not to exist would be a flaw in it? Can certain ideas prove actual circumstances? Can an idea tell us about reality?