Just because something gains existence doesn't make it more valid over other existences...
Not more valid, no. Equally valid, though, yes.
Blade said:
... just like how the Occuria claimed they ruled History isn't true, neither is the idea that one world can take over reality compared to another world that existed before.
There's no similarities in the things you're comparing. One doesn't exist (veracity in the Occuria's arrogant and entitled belief), one does exist (the inhabited world that is Ivalice).
Blade said:
This world is not less important than Ivalice... this world existed BEFORE Ivalice and that means this world takes priority!
Even if you hate this world, that's no justification for disassociating with it or wiping it out... just as it was not up to Mewt to decide the fate of this world when he took control of Ivalice. That sort of action is very selfish, in fact.
This exact same thing can be seen between Gaia and Terra in FFIX. Just because Terra had advanced technology or Magic doesn't mean it has the right to absorb Gaia in to itself for the sake of preserving whatever limited existence Terra gained.
This is actually a valid comparison, but one that makes the opposite case from what you're proposing.
Terra attempted to preserve itself by assimilating Gaia, as it had done to worlds with very young, fresh crystals. The older, dying world was too weak to eat a world in its prime, however, and so the process was incomplete. It still initiated and partially took place before the malfunction, though.
Both planets came to occupy the same physical space, and while the effects on Terra were relatively benign (i.e. being shifted inside of the other planet while most of its structures were left on the surface), Gaia experienced cataclysm. All of civilization was destroyed and untold lives were lost.
New life -- and altogether new lifeforms -- emerged from this planetwide violation, however, including the various races with animal traits that populate Gaia's world alongside the humes.
What Marche was wholly willing to do to Mewt's created world is comparable to someone deciding to undo the partially successful Fusion spell that merged Terra into Gaia in the blind hope that what used to be there would be restored in exchange for the almost certain destruction of all that is there now.
Even if we took the loss of humes out of it (for which we have no guarantee that we should), would it be anything less than dickish behavior to destroy Alexandria, Lindblum, Conde Petie, what's left of Burmecia, etc., kill Hippaul, Puck, Freya, Fratley, Quina, Stiltzkin, Mogmi and Moguta (and their kids), Doctor Tot, Benero and Zenero, along with all the rest of their kinds and any other non-hume races I've overlooked -- just in the hope that the old civilization
might be restored?
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Re:the Occuria
Occuria created the Espers and the Grimoires the Grimoire created Ivalice. This has nothing to do with Mankind directly... they just participated in the end resulting conflicts related to what the Occuria did. Magic exists in Mewt and Marche's world BECAUSE of the Occuria. The Occuria created Magic as a means to manipulate MANKIND, that includes the creation of Ivalice.
I don't think we've ever been given any indication of the Occuria creating those books. If anything, the implication of Ivalice being created from a Grimoire suggests the opposite. The Occuria are inhabitants of Ivalice. If Ivalice was created by a Grimoire, then the Occuria were most likely created by that Grimoire as well, before they went on in turn to create the Scions.