In the current review (July 20th) for William Faulkners, As I Lay Dying in the review section one of Addie's five children, Vardaman, says after she dies, that he believes she is a fish.
When he said this Vardaman had caught a fish and then he cut it all up into little pieces; once it was cut up, it was no longer a fish.
So it was a fish, and then it was not a fish. Or, as Vardaman sees it, it was a fish, and now it’s a not-fish.
Addie was his mother, and then she was not his mother, the same as the fish changed, so she must be a fish.