We must distinguish in the first place between a plagiarist and a copyright infringer. They are both copycats, but the latter is trying to appropriate revenues generated by property that belongs to someone else—namely, the holder of the copyright on the work that the infringer has copied. A pirated edition of a current best seller is a good example of copyright infringement. There is no copyright infringement, however, if the "stolen" intellectual property is in the public domain (in which case it is not property at all), or if the purpose is not appropriation of the copyright holder's revenue. The doctrine of "fair use" permits brief passages from a book to be quoted in a book review or a critical essay; and the parodist of a copyrighted work is permitted to copy as much of that work as is necessary to enable readers to recognize the new work as a parody. A writer may, for that matter, quote a passage from another writer just to liven up the narrative; but to do so without quotation marks—to pass off another writer's writing as one's own—is more like fraud than like fair use.
"Plagiarism," in the broadest sense of this ambiguous term, is simply unacknowledged copying, whether of copyrighted or uncopyrighted work. (Indeed, it might be of uncopyrightable work—for example, of an idea.) If I reprint Hamlet under my own name, I am a plagiarist but not an infringer. Shakespeare himself was a formidable plagiarist in the broad sense in which I'm using the word. The famous description in Antony and Cleopatra of Cleopatra on her royal barge is taken almost verbatim from a translation of Plutarch's life of Mark Antony: "on either side of her, pretty, fair boys apparelled as painters do set forth the god Cupid, with little fans in their hands, with which they fanned wind upon her" becomes "on each side her / Stood pretty dimpled boys, like smiling Cupids, / With divers-colour'd fans, whose wind did seem / To glow the delicate cheeks which they did cool." (Notice how Shakespeare improved upon the original.) In The Waste Land, T. S. Eliot "stole" the famous opening of Shakespeare's barge passage, "The barge she sat in, like a burnish'd throne, / Burn'd on the water" becoming "The Chair she sat in, like a burnished throne, / Glowed on the marble."
Mention of Shakespeare brings to mind that West Side Story is just one of the links in a chain of plagiarisms that began with Ovid's Pyramus and Thisbe and continued with the forgotten Arthur Brooke's The Tragical History of Romeus and Juliet, which was plundered heavily by Shakespeare.
in Paradise Lost plagiarized Genesis, as did Thomas Mann in Joseph and His Brothers. Examples are not limited to writing. One from painting is Edouard Manet, whose works from the 1860s "quote" extensively from Raphael, Titian, Velásquez, Rembrandt, and others, of course without express acknowledgment. Milton
If these are examples of plagiarism, then we want more plagiarism. They show that not all unacknowledged copying is "plagiarism" in the pejorative sense. Although there is no formal acknowledgment of copying in my examples, neither is there any likelihood of deception. And the copier has added value to the original—this is not slavish copying. Plagiarism is also innocent when no value is attached to originality; so judges, who try to conceal originality and pretend that their decisions are foreordained, "steal" freely from one another without attribution or any ill will.