Two centuries after Shakespeare, not to mention ten times as many after Homer, people still did not know what "poetry is."…

So it is hardly surprising that, after the mere 75 years in the history of jazz, this latter-day art form still tempts the definers. In "Jazz Is" a long-time social critic and heart-on-sleeve jazz specialist throws the latest lifeline to a laity left floundering by such wry semi-truths as the one attributed to jazz's Shakespeare, Louis Armstrong: "If you have to ask what jazz is, you'll never know."

Nat Hentoff is a kind of prototype of the white kid who hung around the dancehall door and never got over the sounds of the black musicians and their musical descendants, now of every color. Today he's in that middle-aged generation which can still look around and find what he calls "survivors of every one of the campaigns" waged by...