Roman Catholics: Teacher of the Pope

Traditionally, papal encyclicals cite only writings from the past: scripture, previous encyclicals, the declarations of church scholars and saints. One of the novelties of Pope Paul's recent Populorum Progressio is that it is studded with references to contemporary works and living thinkers. To students of Paul, it came as no surprise that his ardent defense of a "true humanism" cited as a source the writings of French Philosopher Jacques Maritain. "I am a disciple of Maritain," the Pope once said. "I call him my teacher."

A Protestant-born convert, Maritain, now 84, has...