>"Must" and "require" presupposes a first principle position, which assumes agreement, right?

It's simpler than that. "Must" (i.e. "imperatives") and "require" (i.e. "necessity" or "need") affect nothing directly outside the imagination hence they are imaginary things.

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In practice: If I am out in the desert I can believe I "require" water or "must" find water (based on the possibility of me dying soon without ingesting water) but reality is simply that I may soon die of thirst without having found water, the universe doesn't care.