Ye of Little Faith

It matters not whether you believe or disbelieve. Reality is not always based on logic; nor, particularly, are the laws of the universe....
The disappearance of John Henderson was most spectacular. It occurred while he was at the blackboard working an example in multiple integration for his ten o'clock class. The incompleted problem remained on the board for three days while the police worked on the case. It, a wrist watch and a sterling silver monogrammed belt buckle, lying on the floor near where he had stood, were all the physical evidence they had to go on.
There was plenty of eye-witness evidence. The class consisted of forty-three pupils. They all had their eyes on him in varying degrees of attention when it happened. Their accounts of what happened all agreed in important details. Even as to what he had been saying.
In the reports that went into the police files he was quoted with a high degree of certainty as having said, Integration always brings into the picture a constant which was not present. This constant of integration is, in a sense, a variable. But a different type of variable than the mathematical unknown. It might be said to be a logical variable—
The students were in unanimous agreement and, at this point, Dr. Henderson came to an abrupt stop in his lecture. Suddenly, an expression of surprise appeared on his face. It was succeeded by an exclamation of triumph. And he simply vanished from the spot.
He didn't fade away, rise, drop into the floor, or take any time vanishing. He simply stopped being there.
The police searched his room in the nearby Vanderbilt Arms Hotel. They turned a portrait of the missing math professor to the newspapers to publish. Arbright University offered a reward of one hundred dollars to anyone who had seen him.
The police also found a savings pass book in his room. It had a balance of three thousand eight hundred and forty dollars, which had been built up to that figure by steady monthly deposits over a period of years. It also had a withdrawal of three hundred and twenty dollars two days before the disappearance. They were sure they were on the path to a motive. This avenue of exploration came to an abrupt end with the discovery that he had traded in his last year's car on a new one, and that sum had been necessary to complete the deal.