# Page 67

Location: Chapter 8, solution to Problem 2. In particular, the seventh paragraph on the page, lines 5-7.

## It is

... or he could be a knight who truthfully claimed that the fountain was on the island and who also claimed that the fountain was not on the island. ...

## It should become

... or he could be a knight who truthfully claimed that the fountain was not on the island and who also never claimed that the fountain was not on the island. ...

## Short explanation

The line of reasoning presented in the book requires the corrections given above in order to comply with the subcase of $B_2$ in which $B$ is a knight.

In this Reddit discussion (dating back to 2014), doubts are raised over an important part of the solution: namely, it is argued that the reasoning in the Case $A_2$ last paragraph of page 68 ("In this case, here is how Reynolds would reason: ...") is flawed.
More precisely, the discussion goes, how could (the hypothetical, case-$A_2$) Reynolds include in his reasoning the fact that the fountain is not on the island, a fact not yet available to him at the time (remember the question asked by Craig is "do you know whether you would have been able, at the time, to draw a conclusion on the fountain, had the other person answered instead?")
But this objection seems questionable: imagine, at present time (the time when Craig and Reynolds discuss), Craig having models of Reynolds-on-the-island in mind. One of these models is the Reynolds of case $A_2$, who, after having gotten the second answer, knows the fountain is not on the island and is able to reason as reported in the paragraph ("The island is not here, both $A$ and $B$ are knights: had $B$ answered, he would have given a negative answer. At the time, just knowing that $B$ gave such an answer, I would have had no chance at finding out the answer. So I am going to tell Craig that yes, I know whether I would've been able to find out -- indeed, I would not have been able to.").