What claim does Taney make in this passage?

As a citizen of Missouri, Sanford can pursue a case against Scott.
The Missouri circuit court does not have jurisdiction in the case against Sanford.
Scott should pursue his case against Sanford in a state other than Missouri.
Scott is a citizen of the United States, but not of the state of Missouri.

London includes a quote about john thornton as he is observing hal attempt to motivate the exhausted dogs "it was idle, he knew, to get between a fool and his folly". if the word "idle" is defined as "of no real worth, importance, or significance", what does this statement mean with regard to hal? who is the fool? what is hal's folly? why would john thornton think it of no real worth or useless to intervene?