Unit 8 · Detecting Reasoning Errors

Trees Around Waterfall — Reasoning Error

Lesson 17 of 34

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About this lesson

Ivy saw the waterfall a month ago but can't remember where she was. Annie says her brother's painting of the lookout view proves he was there more than two months ago. One of them is right — but the other is making a sneaky hidden assumption.

Chapters

0:00Introduction
0:21Question
1:14Strategy
1:57Step 1: Lock in the facts
2:19Step 2: Ivy — correct elimination
2:48Step 3: Annie — hidden assumption + answer
3:35Correct answer

Question

Because of all the trees around it, the waterfall at Blake’s Point can only be seen from two places: Dwarda’s Crossing, or the lookout across the valley. It’s been impossible to get to the lookout since the rope bridge fell down two months ago.

Ivy said: “I remember seeing the waterfall about a month ago, but I can’t remember where I was. It must have been Dwarda’s Crossing.”

Annie said: “My brother did a beautiful painting of the view of the waterfall from the lookout. That must have been more than two months ago.”

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

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