Like most others who have posted, music and lyrics are organically one for me. That may depend on whether a songwriter "writes" songs on an instrument. I rarely do. If you're sitting around plunking on a banjo or picking out a new melody on guitar, then the song may flow from that. I'm usually driving my car, and so far I haven't gotten any melodies out of my steering wheel. I find it more free-wheeling (blame that one on Art Thieme) to write songs without an instrument. That way I'm not limited by whether or not I can comfortable play the tune on an instrument or figure out all the words. Plenty of time to deal with that after the song is written.