The evidence of experience. I mean if I could find one example where this truth is wrong, then that would disprove it. Haven't been able to.
Beyond that I can't, because if there was a standard that you could compare this to, or a light you could hold it up against, then that would be the answer, and the former would be disproven.
So it is appropriate that you can't actually have evidence to prove this. This truth is what everything else is judged by.
So it can't be proved, only disproven. Or to say it another way, to prove or disprove it are the same thing, they both disprove.
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