"But how can you possibly know that time traveling has never been done?" the chemist protested. "Someone from our future may have gone into the past many times." "I should think they'd have created quite a commotion," the lawyer observed. "Wouldn't we have heard of it from our historical records?" "Of course." The chemist was smiling now. "We probably have. History tells of many important occasions on which a 'vision' appeared. A miraculous presence, such as Joan of Arc, for instance, or the Ang