A reader writes:
"Since the whole gravity thing is just a theory, we should have tethers so we don't float out into space."
This may make my point better than I did. Knowledge about the real world consists entirely of theories, which we have more or less evidence for. We can't prove, absolutely, that gravity will always work—because absolute proof belongs to abstract subjects, like mathematics. But the evidence from our past experience is so strong that we are quite willing to walk around without tethers.
