Why is the argument about Moby Dick being an animal an example of affirming the consequent?

The premise that the argument about Moby Dick is an example of affirming the consequent is correct. In formal logic, this is a formal fallacy where one incorrectly assumes that because a conditional statement is true, the reverse must also be true.12

The structure of the argument is as follows:
* If p (Moby Dick is a fish), then q (Moby Dick is an animal).*
* q (Moby Dick is an animal) is true.
* Therefore, p (Moby Dick is a fish) must be true.*

This is invalid because the truth of the consequent (q) does not guarantee the truth of the antecedent (p). While it is true that all fish are animals, not all animals are fish. By affirming the consequent—that Moby Dick is an animal—the argument fails to prove that he is specifically a fish, as he could belong to another category of animal (such as a mammal, which is the case for a whale).23