My reason for believing…
My reason for believing that the universe did not come into existence just now is that I remember it being in existence yesterday.
You might claim that this is a bad reason, because it is possible for the universe to have come into existence just now with a lot of false memories in place. I’ll grant that it is possible for that to have happened. But so what? Pointing out a mere metaphysical possibility proves very little about my actual epistemic state. Unless you have some reason to offer for treating my memories as globally deceptive, or even profoundly unreliable, you have not yet given me any reason for doubting the evidence of my memory. But without some such reason, you’ve given me no reason to entertain your “might have been” as a “maybe” worth considering.
Cf. Outwitting Old Nick for fuller remarks on the older version of the same puzzle.
Anticopyright.