However strongly held today, our current concept of plagiarism (and, relatedly, copyright) has evolved over time, and emerged largely in the eighteenth century and later; earlier writers worked very differently, and may even have seen borrowing from their predecessors as ideal.
Plutarch was to Montaigne what Montaigne was to many later readers: a model to follow, and a treasure chest of ideas, quotations, and anecdotes to plunder. “He is so universal and so full that on all occasions, and however eccentric the subject you have taken up, he makes his way into your work.” The truth of this last part is undeniable: several sections of the Essays are paste-ins from Plutarch, left almost unchanged. No one thought of this as plagiarism: such imitation of great authors was then considered an excellent practice. Moreover, Montaigne subtly changed everything he stole, if only by setting it in a different context and hedging it around with uncertainties.Bakewell, How to Live, page 66
In professional contexts, I consider plagiarism unethical at worst, sloppy at best. But I wonder, with copying made easier—automatic, even—if that idea can hold for long. If plagiarism was good enough for Montaigne, maybe we shouldn’t be so quick to dismiss it. (But then, I don’t know.)