"Because gravity brings things downward." Dan, thankfully, is very good at not showing on his face or in his quiet, grating voice that he thinks Boimler's missing a critical fact so obvious that Dan feels thrown to have to mention it. He's doing his best to be respectful and open-minded as Boimler throws some religious silliness at him.

Really, it's not that different from other dogma about plagues of locusts or speaking in tongues; it requires faith in something that Dan finds just objectively insane. The idea that the Earth's a giant ball held together by some mystical sideways-gravity that also, somehow, tricked everyone into thinking they were standing upright even when they were upside down is ludicrous on its face.

"Do you think that's how it works in this world?" he asks, keeping his skepticism very suppressed.