The discussion of Bob Dylan and his "borrowing" habit goes on (especially on the Internet), but not usually at a high level of sophistication. Part of the problem is that people in general are not working with a clear notion of what plagiarism is. I would like to submit some criteria for understanding the notion of plagiarism and the conditions under which it can be identified.
The minimal necessary condition for plagiarism is the uncredited use of someone else's work. I think everyone can agree that if this condition is not present, then plagiarism is not present. However, note that this is a necessary condition, not a sufficient condition. By itself, uncredited use is not plagiarism. One might allude to another's well-known work, without a citation and without plagiarism. Faulkner did not need to credit Shakespeare for the title of The Sound and the Fury; he could assume that the allusion would be recognized. We can sing the song "Happy Birthday" without crediting Patty and Mildred Hill every time. We might also unconsciously or forgetfully incorporate someone else's words into our own composition; when someone is accused of plagiarism, this is usually the first line of defense.
The last point leads to an addition to our first necessary condition. There must also be an intention to present the uncredited work as your own work. That's why an effective defense can be "I forgot to give credit" or "I used this unconsciously"; it denies that the use was intentional. Every teacher has heard this defense from students accused of plagiarism, and sometimes it might even be true. It was George Harrison's (unsuccessful) defense against his plagiarism of "He's So Fine" for "My Sweet Lord."
It might seem that these two necessary conditions, uncredited use and intention to mislead, are jointly sufficient criteria for plagiarism. Nevertheless, I want to add one more condition. The question is often raised whether older authors, like Chaucer or Shakespeare, were guilty of plagiarism. Chaucer, for instance, used extensively Boccaccio's Il Filostrato in composing Troilus and Criseyde. Many of Shakespeare's plots come from Raphael Holinshed's Chronicles. Going even farther back, the gospels Matthew and Luke made use of passages taken almost verbatim from the Gospel of Mark. Are all these writers plagiarists? I think we feel intuitively that the answer is no, although our definition as currently constructed would imply that the answer is yes.
Therefore we need to add the further condition making reference to a presumption of originality. During the time of Chaucer, or Shakespeare, or the gospels, there was no overarching presumption that a book or play contained only original work. The cultural norms were different; but at some point between Shakespeare's time and our time (indeed, already in the 18th century) a new assumption of authorial originality became normative, at least for written compositions.
So our concept of plagiarism has these three components: (1) uncredited use that is (2) intentional (3) within a presumption of originality.
The problem with Dylan's borrowing habit is complicated. He and others have often made reference to a "folk process" in the arena of folk music or popular music, wherein the older, pre-modern, practice of free borrowing and re-use of older work was still normative. Hence the many tunes and lyrics derived from other artists found in his catalog are widely felt (possibly correctly) to be non-problematic (although Dylan is unwilling to extend this permissiveness to artists borrowing his work). In other words, there is no strong presumption of originality in folk music — or possibly in music in general (although there has to be at least a weak presumption of originality in order to copyright anything).
However, there is a much stronger presumption of originality in written work published under one's own name. That's why, in the many passages in Chronicles, Volume 1 where Dylan has lifted from other authors, without credit or credible expectation of a recognizable allusion, he is arguably guilty of plagiarism.
I'm not trying to tear down the whole body of Dylan's work. I don't want to—I'm a fan—and I couldn't even if I did want to. However, I do think that in the last 10 years or so he has compensated for the waning of his creative powers by over-indulging in this borrowing habit, which reaches a high point in his own autobiography. It's not pernicious in the sense that any living person is harmed by it, but it should be acknowledged for what it is.
BIBLIOGRAPHY: Besides the links given above, see Scott Warmuth's Goon Talk (passim). See also the relevant chapter in David Yaffe, Like a Complete Unknown. Their take is different than mine.