Ishmael expands on the Biblical story of Jonah and the whale. He explains that many Nantucketers don’t really believe the story in all its detail.
One whaler in particular, a man from the port of Sag-Harbor who was called by the name of his hometown, distrusted the story because the illustration in his Bible showed a whale with two spouts, and the whaler knew that only right whales have two spouts.
This man, "Sag-Harbor," also objected to the story because he thought the whale’s gastric acids would have eaten away at Jonah. Ishmael suggests perhaps Jonah’s whale was dead, or something other than an actual whale.
Sag-Harbor’s final objection to the story is that the whale swallowed Jonah in the Mediterranean and spat him out near Nineveh (a city in ancient Assyria, now Iraq), which is more than a three-day journey. Ishmael suggests the whale might have gone around the Cape of Good Hope instead of across the Mediterranean… which is a longer route anyway!
Since Ishmael can’t refute Sag-Harbor’s arguments, he condemns the man for having too much "pride of reason" (83.5).