Sometimes, in moments of bewilderment or happiness, you may catch yourself singing or whistling a song whose words, on reflection, explain how things really are with you—"Good Day Sunshine" when it's raining, "Rain" when it's sunny, "Hello Goodbye" when you don't know whether to stay or go. In all kinds of weather and situations, the song interprets you….

To the sound of the slow-motion footfall of acoustic guitar and bass, Van Morrison's radiant and archetypal vision of a woman on her horse of snow has recently brought back to me perhaps the most extraordinary of these mysterious songs, "Slim Slow Slider," the three-minute blues reverie that concludes the time-less Astral Weeks album recorded in 1968.

Coming as it does after "Madame George" and "Ballerina"—with their almost Blakean commitment to the themes of sexual organization, childlike vision and angelology—"Slim Slow Slider" can certainly be taken simply as...