When is the last time you heard a liberal describe himself as a "liberal"? It’s probably been a long time. These days, those on the left are more likely to call themselves "progressives." Writing in The New York Times, Columbia University economist Jeffrey Sachs said there have been two progressive eras — one in the early 20th century and the second under Franklin Roosevelt. He called on modern liberals to usher in a third era. But what exactly is "progressivism"? To many people, the term "Progressive Era" evokes fond caricatures of Teddy Roosevelt and such reforms as safe food, the...