The words come first. The melody is a "setting" of a previously written usually canonical poem.
The process of song setting is not collaborative.
Words and music are fundamentally at odds. The composer overrides the poet's art.
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Reverse this, and you get the "organic" model. Neither words or music are essentially prior, and either might come first as a matter of practice.
Songwriting is often collaborative, or can be done by one person writing both words and music.
The listener experiences the words of the song as "the words of the song," not as a melodic imposition on a previous text.
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Neither model is correct in absolute terms, but clarity emerges if we contrast the two models. The second, vernacular paradigm has more far-ranging applicability than a model pertaining only to Western art song.