“I say, Holmes, how did you know that the crucial evidence would be in the galley of the yacht?”
“It was an elementary inference, Watson. As you were so quick to point out, the locked room showed that the murderer could not possibly have committed the crime and escaped. Yet the body of the victim and the absence of murderer showed that they had done so. I was puzzled until I remembered that everything follows from a contradiction, and this allowed me to conclude that the crucial evidence would be wherever I looked.”
“I see,” I said, although I really did not see. “But why the galley of the yacht?”
Holmes looked at me as if I were missing the obvious. “Because I was hungry. If I could find the evidence anywhere, then I might as well find it somewhere I could also make a sandwich.”
“Right then! But what about relevance constraints on logical consequence?”
“Watson, you disappoint me. If there were relevance constraints on consequence, then I could not have solved the crime. I did, so there are not.”
Then I realized that I, too, could derive anything from the contradiction Holmes had exploited. So Holmes conceded that I was clever, poured me a cup of tea, and left me alone for the rest of the afternoon.