[hnfong]

Don't know why you're downvoted so much, but your observation is spot on.

This is essentially Descartes evil demon issue. If you can't disprove that an evil demon (with god-level powers) is deceiving you at everything you perceive, then how are you going to be sure about anything? (including that the Earth is not flat?)

It has always been a difficult philosophical issue about how much we can trust reality itself.

[Forgeties79]

That’s an interesting big picture philosophical question with big picture implications, but it’s not really what we are talking about here. We are confirming the things we can about our shared reality, not questioning its very nature.