Wednesday night just before retiring he had another disturbing thought. He had explained the theory to his son. But that had been weeks before, and Fred was steeped in the mechanism of unbelief. Good thing, or he might have been the first to disappear. "What's the matter with you, Martin? Can't you even answer when--" The rest of what his wife was saying faded in the startled realization that he was eating dinner. "Sorry, dear," he murmured. "I was thinking." He was trying to recall something that might tell him what day it was. It was obviously evening or they wouldn't be eating dinner. "Uh," he said casually, "what day is today?" "Saturday," Fred said. "Now Fred, don't tease your father about his absent-mindedness. This is Thursday." Thursday! That was right. He had given the lecture on the necessity of unbelief today. There was tomorrow, when he could see if any of the class had disappeared yet. He couldn't be certain, of course. Just because a student didn't show up didn't mean he or she had vanished. He fixed his eyes on Fred, across the table, and smiled. Fred, at least, was a source of comfort. He knew the theory and hadn't vanished. "Dad," Fred said. "I've been wondering if you saw a point of similarity in the two disappearances?" Martin thought, good heavens, does he have any inkling of what I've been thinking? Of course not! He's just fumbling. Better to discourage him. "Sorry, son. There aren't any similarities except accidental ones. I've had the confidence of the police on this. The cases are quite unrelated." Fred refused to be sidetracked. "Dr. Henderson's face lit up as though a sudden idea had struck him. I talked with some of his students. That's what they all thought. And Horace Smith shouted to his wife, 'Ethel! I've got it!' The next instant in each case they vanished into thin air." "But that doesn't mean a thing." * * * * * In the privacy of his study Martin Grant allowed himself to become excited. Fred had unwittingly come upon the vital clue to the two disappearances. "Let's be clear about this," he said to himself, drumming on his desk nervously with his fingers. "Undoubtedly there's a connection between the vanishing and my theory. Both Horace and John arrived at something I've missed. And since my theory is exhaustive it can't be there. It must be--yes--it _must_ be that they went a step farther." He pondered this a moment and added grudgingly, "A step I have missed." Then even more grudgingly, "An obvious step." Automatically he opened a drawer and brought out a sheet of paper and a pencil. He wrote: _The theory contains within itself the proof that the universe must, by logical necessity, be constructed according to said theory. But observation and experience say this is not true._