Is Maya home?

About this video

Leo wants to know whether his friend Maya is at home, so he walks past her house. Her bike is outside, the light in her bedroom is on, and when he knocks he hears her voice: "Coming!" Each of the three fits the idea that Maya is home. But do they all count the same?

In Knowledge Level 1, evidence was strong when nothing else could easily explain it, and weak when many things could. Here is a sharper way to ask the same thing: how much more would I expect to see this if the claim were true than if it were false? Maya leaves her bike outside when she is home, but also when she walks to the shop, which she often does. Leo would see the bike either way, so it points only a little. If she is home in the evening her light is usually on, and if she is out it is usually off, so the light points quite a lot. If she is home Leo hears her, and if she isn't he almost surely doesn't: her voice points a great deal.

This catches a common mistake: thinking a piece of evidence counts just because it fits an idea. Leo's sister says Maya must be home, because her house is still standing. It fits, but the house would be standing whether Maya is home or not, so it points nowhere. A fortune teller who says "you have had some difficult times" says something that fits everyone. A clue you'd see twice as often if the claim were true moves you a little, one you'd see a hundred times as often moves you a lot, and several independent clues pointing the same way add up. This video goes with the How Much Does It Point? lesson of Knowledge Level 2 on Knaph.

Transcript

1. 0:00Leo walks past Maya's house. Her bike, her light, her voice. Is she home?
2. 0:05Ask how much more you'd expect this if she were home than out.
3. 0:11Her bike is outside either way. It points only a little.
4. 0:17Her light points quite a lot. Her voice, a great deal.
5. 0:23Her house is still standing! It fits, but you'd see it either way.
6. 0:29Would you see it either way? Then it points nowhere.