"Ain't No Mountain High Enough" refuses to acknowledge limits. Marvin Gaye and Tammi Terrell recorded it as a duet in 1967, two voices agreeing that no obstacle in the physical world can keep them apart. Mountains, valleys, rivers, darkness -- the song lists every barrier nature can produce and swears each one can be crossed. The hyperbole is deliberate. Love that does not sound slightly unhinged is not love worth singing about. The promise is not realistic. It is romantic. That is the whole point.