It is a good thing to start with a definition of song. Of definitions I have seen that of Bourgault-Ducoudray's appeals to me. He says that a song “applies simple, spontaneous, rich form to a great poem.” “Simple” seems to negate “rich,” though I think we can guess what he means; “spontaneous” is the right word; what we all feel about the really successful song is that it comes to us in one gush straight from the composer's brain (even if that isn't the least the case). The words should be “great” if only from the singer's point of view; unless they are, he cannot sing with complete conviction. Mendel's Lexicon adds that a song “presents a single emotion to mind and feeling.” That is worth insisting upon; Schubert took a long time to find out how to give his song that unity which became its conspicuous merit.