tl;dr- The migration was a mistake. I flagged it to be migrated back, but the migration back was rejected. This is weird because:
Both SE.Philosophy and SE.Math have Meta-questions about this exact issue, and they both agree that this question belongs at SE.Philosophy and not SE.Math.
This is clearly a Philosophy question from a Philosophy textbook discussing Philosophy; it has nothing to do with Math.
Even if the question had something to do with Math (and, again, it doesn't), it'd still be inappropriate to migrate it since it's a Philosophy question and thus on-topic here.
Even if the question didn't have something to do with Philosophy (and, again, it does), it'd still be inappropriate to migrate it since it's not a Math question and thus off-topic at SE.Math.
Despite all of this, for some reason, it was originally migrated and SE.Philosophy has rejected the migration back.
Forward: This is weird enough to be interesting!
In case anyone feels criticized here, please know that I don't mean to be overly critical! It's just this is a very strange situation, which makes it interesting.
Here's a timeline:
2019-09-04: The question was asked.
2019-09-05: The question was migrated by this point, from SE.Philosophy to SE.Math.
2019-09-05: This SE.Philosophy.Meta question was posted asking about why it was migrated.
2019-09-10: I flagged the question on SE.Math to be migrated back here.
2019-09-13: The question was closed as off-topic at SE.Math and a migration back was attempted.
2019-09-13: The migration back to SE.Philosophy was rejected.
2019-09-13: This SE.Philosophy.Meta answer was posted.
The migration from SE.Philosophy to SE.Math wasn't appropriate.
The migration from SE.Philosophy to SE.Math wasn't appropriate because:
SE.Philosophy.Meta has already declared the question on-topic.
SE.Math.Meta has already declared the question off-topic.
The question is a Philosophy question.
The question isn't a Math question.
A migration would make sense only in the opposite case.
1: SE.Philosophy.Meta has already declared this on-topic.
There's an existing SE.Philosophy.Meta question on this exact issue,
which currently only has 1 answer, with +2/-0 votes, stating:
If the logic question is essentially non-numeric in nature, philosophy it is.
–@VishwaJay's answer to "Do logic questions belong in Philosophy SE or Math SE?"
Since this question has nothing to do with numbers, the existing SE.Philosophy.Meta position is that it belongs here.
2: SE.Math.Meta has already declared this off-topic.
There's an existing SE.Math.Meta question on this exact issue,
which currently has two answers. The top-voted answer (with +6/-0) states:
Questions that are not amenable to mathematical techniques are not on topic here. These might include questions on pure philosophy or questions that are purely historical.
–@CarlMummert's answer to "Should Logic be queried on Maths SE or Philosophy SE?"
Since this question isn't amenable to mathematical techniques, it's not on topic at SE.Math.
3: This is a philosophy question.
The question is clearly a Philosophy question because:
It's about the content of a Philosophy textbook, "The Norton Introduction to Philosophy" (2015).
It's a logic question of the sort students would do in a Philosophy class in highschool, or maybe an intro-level Philosophy 101 class in college.
It's about basic reasoning, which is a Philosophy topic.
4: This isn't a Math question.
This isn't a Math question because:
It has nothing to do with Math.
It's not from a source that discusses Math.
It's not a topic taught in Math classes.
5: The question shouldn't have been migrated even if it could've fit on SE.Math.
Questions can be migrated when they:
don't fit on the current stack; and
do fit on the migration-target stack.
Either one alone isn't sufficient, and this question fails both migration criteria.
Conclusion: The question should be migrated back to SE.Philosophy.
Since it's a Philosophy question that was posted to SE.Philosophy, it belongs on SE.Philosophy and should be migrated back here from SE.Math, where it doesn't belong as it's not about Math.
Additionally, it'd be nice if a mod could explain what happened. I mean, this contradicts the Meta guidelines for both sites, plus it's just weird to see a Philosophy question from a Philosophy textbook that has nothing to do with Math rejected from SE.Philosophy and pushed to SE.Math.
Which I hope no one feels too bad about! It's just a weird situation that'd be nice to see an explanation of.