Sure, I can "prove" that my sheet of sandpaper works by using it all day. Does that mean it should work all day tomorrow? It's a useful bit of information ... a

Sure, I can "prove" that my sheet of sandpaper works by using it all day. Does that mean it should work all day tomorrow? It's a useful bit of information ... about how it's approaching the end of its service life. If you don't evaluate how much sand is left before using it again, what is there to debate? Either you don't care if it works tomorrow, or you're about to learn an important lesson on how sand paper works.