That part of a work of one author found in another is not of itself piracy, or sufficient to support an action; a man may adopt part of the work of another; he may so make use of another’s labors for the promotion of science and the benefit of the public. – Lord Ellenborough, quoted in Bouvier’s Law Dictionary by John Bouvier, 8th

I suppose every old scholar has had the experience of reading something in a book which was significant to him, but which he could never find again. Sure he is that he read it there; but no one else ever read it, nor can he find it again, though he buy the book and ransack every page. – Ralph Waldo Emerson