Speaking of "hearing" an as yet unwritten melody alanabit, McCartney literally dreamt up "Yesterday" waking one morning with the melody going through his head.A very cogent analysis of what may have been an unconscious scource of inspiration for this has been published at
http://www.Geocities.com/hammodotcom/beathoven/index.htm.
If anybody who is less blue_clicky challenged than I would like to do something with that I'd be grateful.
B.T.W. even if you dislike The Beatles, anyone who is interested in creaivity in music composition will probably like that site.