The much-discussed difference between the modern and postmodern eras may come down to this: Modernity will be remembered by the slogans of its philosophers, whereas postmodernity will be remembered by the slogans of its advertisers.

"I think, therefore I am" defined the era of Descartes; "Just Do It" and "Obey Your Thirst" define the era of Nike and Sprite. Descartes' rallying cry was a declarative statement of proof, but advertisers these days are in an imperative mood, borrowing their tone from the '60s bumper sticker that ordered us in no uncertain terms to "Question Authority."

I started making a list of these exhortations back when Gap insisted "Everybody in Khakis," and the list keeps getting longer—Nike, for example, has recently instructed us to "Run," "Live Strong," and "Make It Personal." But none has captured my imagination quite like a sign in Starbucks, that laid-back emporium of java and earth tones. Placed under a rack of Starbucks compilation CDS, the sign offered this postmodern commandment: "Live More Musically."

Maybe the sign caught my eye because a few years ago I decided to return to my own musical roots, revisiting in my 30s the classical music I had practiced as a child. I've spent countless hours since working my way through Bach's Well-Tempered Clavier, a beautiful and difficult collection of preludes and fugues that has taught me perseverance, humbled me with my own limitations and laziness, and given me a few glimpses of grace.

But even for nonmusicians, Starbucks' sign seems to point in the right direction. To live more musically implies having a rhythm, a sense of harmony, a melody that gives shape to life. Who wouldn't want to live more musically?

As postmodern slogans go, "Live More Musically" ...