And so I did. Because I figure if I ever get sent to Hell, it might well involve dealing with plagiarism in one form or another--which I here define as the use of others words and specific ideas without acknowledgement. I spent years teaching research and writing in higher ed, worked in training units where we produced training materials, did a plagiarism webinar, wrote papers, worked to persuade other faculty into making first events teachable moments rather than range wars, and learning from a student who bought a paper that she because she bought it she could use it however she pleased--which was to submit it for a grade.
But back to Mark and Helen. In this letter, Twain declares that most everything in the human condition comes from our shared pool of ideas:
"...and there is not a rag of originality about them anywhere except the discoloration they get
his mental and moral calibre and his temperament, and which is revealed in characteristics of phrasing"
And it is the "characteristics of phrasing" that invokes the great plagiarism tempest--the fixed form the idea takes. Copyright law protects a persons right to make a living based on those fixed forms, and academics base their value on the contribution of both ideas and words (which is nicely acknowledged and shared via Creative Commons).
Twain was a man unafraid to wrangle with trying circumstances or recognize an albatross for what it was-- and at times teaching of appropriate acknowledgement certainly falls into either category. I agree with Twain the ideas we express very likely come from a long line of others' ideas. And I support saying where they come from, an idea which communicates our shared history.