The cancelled trip

About this video

Yusuf gets a message from his classmate Elif: "The school trip tomorrow is cancelled!" Elif is usually right about school news, so Yusuf believes her. Then one of two things happens. In the first story, the teacher sends an email: "See you all at the bus at 8 tomorrow!" In the second, Yusuf looks at the calendar and sees that today is 1 April, the day people play tricks on each other. Either way he stops believing the trip is cancelled, but the two stories work differently.

A new fact that takes away a reason you had is called a defeater; the American philosopher John Pollock studied them closely in the 1970s. There are two kinds. Evidence against points the other way: the teacher's email says the opposite of Elif's message, and it is stronger, so it wins, and Yusuf should believe the trip is on. A reason that stops counting says nothing about the trip at all: the calendar only shows that Elif's message may be a joke, so it is no longer a good reason. Yusuf is back where he started. He should suspend judgement and find out, perhaps by asking the teacher.

People often mix these up. When a reason turns out to be bad, they jump to believing the opposite: the man who said it would rain is a liar, so it won't rain. But a bad reason for rain isn't a good reason for no rain. And a reason can come back: if Yusuf remembers that Elif hates April tricks, the calendar stops mattering and her message counts again. This video goes with the When a Reason Is Taken Away lesson of Knowledge Level 2 on Knaph.

Transcript

1. 0:00Elif says the school trip is cancelled. She's usually right, so Yusuf believes her.
2. 0:05Story one. The teacher emails, see you at the bus. Evidence against.
3. 0:11Story two. Today is the first of April. The message may be a trick.
4. 0:17Evidence against, believe the opposite. A reason that stops counting, you just don't know.
5. 0:23But Elif hates April tricks. Now her message counts again.
6. 0:29Does the fact point the other way, or take a reason away?