Where does one draw the line between honoring the work of an earlier writer/scholar/artist and plagiarism or fraudulent re-use?

Surely intent to deceive has something to do with this. If I set out to use X's ideas in order to solve a problem, and I make it clear that is what I am doing, then that is honouring. If I don't make it clear that is what I am doing, nor could I reasonably expect that all my readers will know this is what I am doing, then that is plagiarism.

A slightly different version of your question would be this: "Where does one draw the line between honouring ... and merely rehashing old ideas?" (I love the word 'rehash', by the way, it being literally visceral.)

We are probably all tempted by the answer: "a work of philosophy (or art, or whatever) is not a rehash if it exhibits some amount of originality." So, suppose I use X's ideas (and I'm clear about what I am doing) to try to solve a problem that X did not consider, or to write a novel about a kind of situation that X did not. That is surely a sufficient degree of originality to avoid the accusation of rehashing.

But originality is not so easy a concept to define. As an educator, undergraduate students often try to produce original work, and end up inadvertently ... making a hash of things. Walking before running and all that. And yet, if the same student merely repeated back to me what some philosopher had said, that would also not be a good result. In this case, a sufficient level of originality is showing that one is able to understand, by putting into different words, explaining very clearly, using different examples, and so forth.

Such a level of originality might earn a very good mark on a undergraduate essay, but wouldn't get one published in a journal. So, again, originality functions differently.

A nice meditation on the concept of originality is the short story "Pierre Menard, author of the Quixote" by J.L.Borges. Look it up!

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