I can say "I believe I can fly!" but then when I see someone try to fly by jumping over a canyon, I'll know I can't fly.
Battler can say "Dammit I believe in witches now!" but then when he sees someone else who doesn't want to accept Beatrice, he can go and say "I don't believe in witches anymore!" because of a new resolve.
(not that great of an analogy, but I came up with it on the spot
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