Oscar: I have been accused of plagiarizing the work of Ethel Myers in my recent article.
Oscar's Conclusion
I didn't plagiarize Ethel Myers in my recent article.
Evidence
Sure I used passages from her book without attribution, but Myers gave me permission to do so, via private correspondence.
Evaluate
The question asks for a principle that justifies Oscar, so Millie's objection is not our concern. Oscar's reasoning has an evidence side and a conclusion side.
Evidence: the author gave permission to use it without attribution. Conclusion: using it that way was not plagiarism / not bad.
We want a rule to connect the two.
Goal
Let's look for a principle like,