The dog in the night

About this video

In a famous story by Arthur Conan Doyle, from 1892, a racehorse is stolen from its stable in the night. Sherlock Holmes points to "the curious incident of the dog in the night-time". The dog did nothing in the night-time, says the policeman. That was the curious incident, says Holmes. There was a guard dog in the stable, and a stranger would have made it bark and wake the stable boys. Nobody heard it bark. So the thief wasn't a stranger to the dog: it was someone the dog knew well.

Holmes used a missing sign: something that would have been there if one explanation were true, and wasn't. People sometimes say that absence of evidence is not evidence of absence, and that is only half right. Whether a missing sign counts depends on one question: would I have seen the sign if the claim were true? Is there an elephant in your fridge? You open it and see none, and that settles it, because an elephant could not hide. Are there germs on your hands? You look and see none, and that settles nothing, because you would never see them anyway.

How you looked matters too. If you searched your whole bag and found no keys, that is good evidence the keys aren't in it. If you glanced into it in the dark, it isn't. So when something is missing, ask: if the claim were true, would this sign be there? And did I look properly, where and when I would have found it? If both answers are yes, the missing sign is real evidence. This video goes with the The Dog That Didn't Bark lesson of Knowledge Level 2 on Knaph.

Transcript

1. 0:00A racehorse is stolen in the night. Holmes notices that the dog did nothing.
2. 0:05A stranger would have made it bark. So the thief was someone it knew.
3. 0:11Would I have seen the sign if it were true? No elephant? Settled.
4. 0:17No germs on your hands? You'd never see them. That settles nothing.
5. 0:23And did you look properly? The whole bag, or a glance in the dark?
6. 0:29Would you have seen it, if it were true?