The soul—let us go further and say the substance, the bulk, the actual and valuable material of all human utterances—is plagiarism.
All ideas are secondhand, consciously and unconsciously drawn from a million outside sources, and daily used by the garnerer with a pride and satisfaction born of the superstition that he originated them; whereas there is not a rag of originality about them anywhere except the little discoloration they get from his mental and moral caliber and his temperament, and which is revealed in characteristics of phrasing.
When a great orator makes a great speech, you are listening to ten centuries and ten thousand men—but we call it his speech, and really some exceedingly small portion of it is his. But not enough to signify. It is merely a Waterloo. It is Wellington’s battle, in some degree, and we call it his; but there are others that contributed. It takes a thousand men to invent a telegraph, or a steam engine, or a phonograph, or a photograph, or a telephone, or any other important thing—and the last man gets the credit, and we forget the others. He added his little mite—that is all he did.
-- Mark Twain
If you're interested in the context behind this letter, I wrote an article (mirror) about it.