By 
Johann Sebastian Bach's Goldberg Variations begin with a quiet aria, follow it with thirty variations, and end with the same aria played again, note for note. On paper nothing has changed. In the ear, everything has.
The architecture
The variations are built not on the aria's melody but on its bass line, a sequence of harmonies that runs underneath every one of them. Every third variation is a canon, in which one voice imitates another at a steadily widening interval. Between the canons come dances, fughettas, showpieces that send the hands crossing over each other, and one long, slow lament in a minor key that sits near the end like a held breath.
It is music about returning, and about how you are not the same when you get back.
How to listen
Do not put it on in the background. Pick a quiet hour, listen to the opening aria twice, then follow the variations in order, noticing which ones make you lean in. By the time the aria returns, you will hear it as a memory rather than a beginning.
Verdict
Few pieces reward attention so completely. Try the variations on piano and on harpsichord; they become two different works, both worth the hour.
