He excuses himself for the earlier stages of the trickster's life by a survey of the border-land between truth and fiction, not by any means a piece of sophistry or cynicism, but a perfectly fair statement of an ethical difficulty which does exist.
There are some people who think that it must be immoral to admit that there are any doubtful cases of morality, as if a man should refrain from discussing the precise boundary at the upper end of the Isthmus of Panama, for fear the inquiry should shake his belief in the existence of North America.

People of this kind quite consistently think Sludge to be merely a scoundrel talking nonsense.

It may be remembered that they thought the same thing of Newman.

It is actually supposed, apparently in the current use of words, that casuistry is the name of a crime; it does not appear to occur to people that casuistry is a science, and about as much a crime as botany.

This tendency to casuistry in Browning's monologues has done much towards establishing for him that reputation for pure intellectualism which has done him so much harm.