The Footsteps at the Lock by Ronald A. Knox

"No," said Bredon as he and Leyland paddled up, it seemed for the fiftieth time, to Shipcote Lock. "I don't find Nigel Burtell's story incredible in the least. I was never at a University, but I can quite understand how a creature of poses like that might experience a sudden revulsion just at the end of his time there. In a small world it must be difficult for a self-conscious person not to pose -- not to wonder what people are thinking of him and whether people are thinking of him; not to impose upon them a false personality if his true personality is not worth imposing. And to leave all that behind must engender a desire to return to the simple emotions. But then, unfortunately, murder is one of the simple emotions; and I shouldn't be really surprised to hear that Nigel had returned to that. He's so confoundedly plausible, you see; I wouldn't put it beyond him to give us a perfectly genuine analysis of his emotions, and then conceal from us the central fact. And it remains certain that he's blown his own alibi to bits. If there was only a hole about the size of a pin's head in the bottom of that canoe, the wind and the stream would carry it no end of a distance before it filled up. And, dash it all, that's all there was. Why shouldn't the murder have been done before the canoe left the iron bridge? And if it was, why shouldn't it have been Nigel that did it?"

"I know, I know. But then, you've always been building such a lot on that argument. To me, the whole thing has been a question of the total time involved."