Unit 8 · Detecting Reasoning Errors

Rob and Paul — Bolivistan Citizens Logic

Lesson 24 of 34

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About this lesson

In Bolivistan, the national team playing always means flags in shop windows. Rob sees no flags and concludes the team isn't playing. Paul sees flags and concludes the team must have been playing. One of them is making a valid logical move — the other is reversing the rule.

Chapters

0:00Introduction
0:25Question
1:09Strategy
2:10Step 1: Write the rule as an arrow — understand what it guarantees
3:10Step 2: Evaluate Rob — the contrapositive (always valid)
3:55Step 3: Evaluate Paul — the converse fallacy (invalid)
5:02Correct answer

Question

Rob and Paul are citizens of Bolivistan. In Bolivistan, whenever the national football team is playing, shopkeepers always decorate their windows with the Bolivistani national flag.

Rob: “That’s funny. I thought the national team were playing today, but I don’t see any flags in shop windows. I must have got the date wrong.”

Paul: “Well, I saw quite a few flags in the shops last Wednesday – that must have been when they were playing.”

If the information in the box is true, whose reasoning is correct?

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