Who knew that the send-up documentary "This Is Spinal Tap" not only lampooned the business of rock and roll but illustrated deep philosophical conundrums? Recall the scene in which Nigel Tufnel, Spinal Tap's lead guitarist and sometime songwriter, sits noodling at the piano, plinking out a few doleful chords.

"It's pretty," comments the faux documentarian (played by Rob Reiner). Tufnel (Christopher Guest) notes that the song he is fooling about with is "in D minor, which I always...