Gregory (Scotland Yard detective): “Is there any other point to which you would wish to draw my attention?”
Holmes: “To the curious incident of the dog in the night-time.”
Gregory: “The dog did nothing in the night-time.”
Holmes: “That was the curious incident.”
What you pay attention to dictates what you’ll miss
The point that the great detective was making was that the absence of something was the evidence which the Scotland Yard detective had overlooked. Holmes of course using imagination and intuition did identify that this was in fact the vital clue. Such a plot device works marvelously well because almost all of us, like detective Gregory, fail to recognise that such an absence is actually ‘there’ in a sense, let alone that it’s important.