How black and white is plagiarism?

Very young children do not distinguish between themselves and the world; they assume that everything belongs to them; only in time and through the conditioning of experience do they learn the distinction between mine and thine and so come to acquire the concept of stealing. The concept of plagiarism, however, is learned in more specialized contexts of practice entered into only by a few; it’s hard to get from the notion that you shouldn’t appropriate your neighbor’s car to the notion that you should not repeat his words without citing him.

- Stanley Fish, Plagiarism is not a big moral deal

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How important is it to include a citation to other people's material which you have used?