As an old Army aviator and having taken too many shots on, from and across unlevel terrain, I have to disagree. There can be many reasons why the inner ear, eye and brain can misinterpret "level", and it has led to many a pilot "flying by feel" and not "trusting" his instruments and to his death. Or in shooting, missing an animal or X-ring, etc. Having spent much time teaching young and old shooters, I've seen many people "think" their rifle sights were level when they were actually canted, and it was only after adding a sight or scope level did they realize how much. YMMV