"Ain't No Mountain High Enough" maps a love that recognizes no natural barrier. Marvin Gaye and Tammi Terrell's 1967 duet is a catalogue of impossibilities made possible by devotion. Rivers do not stop. Valleys do not deter. Darkness does not hide. The song builds a world where love operates as its own law of physics, overriding every limitation the physical world imposes. The hyperbole is not exaggeration. It is a definition. That is what love is, or it is nothing.