Ahh, I was wondering about this the other day myself.
For the most part, I'm content to accept the possibility that the worlds in Final Fantasy are simply altrernate realities from our own, or in some cases, our very same planet in the far distant future.
What would be interesting, is that, if each Final Fantasy was based on the very same planet, at far different times from each other.
If you wanted to put each Final Fantasy on the same timeline, what order would you put them in, in order of past to future?
I'd probably go: FFI, II, III, IV, V, VI, IX, VII, VIII, X; mostly based on the technology and level of civilization and culture present in the game.