The restaurant bill

About this video

Mira and Jon have dinner with two friends, and the bill comes. They agree to split it equally and add a tip, and each works out the sum in their head. Mira says 43 each. Jon says 45 each. Both are good at sums, both are paying attention, and both have seen the same bill. What should Mira think now, and what should Jon?

Mira and Jon are equals in this question: they have the same information and are just as good at this kind of thing. Philosophers call people like this peers; the American philosopher David Christensen used this restaurant bill in 2007. Mira could think: I worked it out, so 43 is right and Jon slipped. But Jon could think exactly the same about her, and nothing she knows makes her own sum more trustworthy than his.

So Jon's answer is a new kind of evidence. It isn't about the bill; it is about Mira's thinking. A peer reaching a different answer is a sign that one of them made a mistake, and neither can tell from the inside which one. Most people feel the answer is clear: both should become less sure, and check it on paper. Some philosophers say a peer's view always deserves as much weight as your own; others think you may sometimes stick to yours.

You may stay fairly sure when you have a reason from outside the disagreement itself: Jon is yawning after a long day, or Mira checked twice, or Jon says 450 each, an answer so far off he must have misread the bill. What Mira can't do is say she is right because her answer is 43. This video goes with the When Friends Disagree lesson of Knowledge Level 2 on Knaph.

Transcript

1. 0:00Mira says 43 each. Jon says 45. Same bill, both good at sums.
2. 0:05They are equals in this question. Philosophers call them peers.
3. 0:11One of them slipped, and neither can tell which from the inside.
4. 0:17So both should become less sure, and check it on paper.
5. 0:23Unless there's a reason from outside, like Jon saying 450 each.
6. 0:29An equal who disagrees is evidence too.