The problem with Robert Plant's solo career is that he has been unable to leave Led Zeppelin's thunderous sound completely behind him. Even without the late John Bonham's gorilla thwack spurring him on or Jimmy Page's demon blues licks chasing his tail, Plant sometimes cannot help resorting to his trademark Promethean theatrics, straining at melodies with salacious vocal jibes and full-moon howls. Fortunately, much of The Principle of Moments finds the singer trying to get around that dilemma by toying with weird hard-rock alternatives and singing in a restrained, though powerful, manner.

One of Plant's best outings on his second solo album is "Big Log," a ballad that features a vague Latin lilt and a discreet synthesizer gloss that complements the sexy elasticity of Robbie Blunt's guitar. Against his best heavy-metal instincts, Plant resists easy histrionics and opts for a far more effective quiet tension in his bluesy wails and bassy coos. Musically, there are several references to the Zeppelin canon here — the jerky "Black Dog"-like rhythm pattern of "Messin' with the Mekon," Blunt's Middle Eastern "Kashmir" — type motif in "Wreckless Love" — but overall, Plant is taking more outside chances.

The experimental half of The Principle of Moments is, in effect, Robert Plant's admission that on his own, he can never improve on Zeppelin's otherworldly he-man fantasies. It is also his declaration of independence from the past — not a denial of it, but just one way of showing there's more to life than "Whole Lotta Love."

“He Will Break Your Heart”

A lightly swinging Latin-influenced, almost cha-cha groove and close harmonies decorated Jerry Butler's early soul hit "He Will Break Your Heart," delivering a stately warning that his rival would never love his girl like he did. The melody came to Butler as he was driving on the highway from Atlantic City, New Jersey, to Philadelphia with Curtis Mayfield, and as Butler told Rolling Stone, "I just sang the melody and Curtis put the chords to it." The song's premise, Butler added, "was something that I'd lived ...The lyric was an experience rather than a revelation. Whereas music is usually a revelation."