[Tutor] Unit testing: Just the API or internal use only methods, too?

[boB Stepp]

Peter Otten, while responding to one of my questions in the past,
mentioned something in passing that apparently has been mulling around
in the back of my head.  I don't recall his exact words, but he
essentially said that I should be testing the public interface to my
classes, but not the methods only used internally by the class and not
meant to be publicly accessible.  Is this generally how I should be
viewing testing?  Would someone be willing to expand at some length on
this topic?

TIA!
-- 
boB