Is the earth round or flat? The problem is that the only way we can find an argument is from a book, but it's almost impossible if you're going to drag a third party out.
Like the existence of a black hole, does it exist? Has anyone found it? It's impossible to know, but Mr. Hocking came up with his theory of black holes, and we're willing to believe it, and we believe in his theory.
No it's not the only way, you dummy. People know the earth is not flat because there are pictures of the earth from space, millions of pictures and hundreds of videos and livestreams, you don't need to read any book to know the earth is not flat.