Modernism is one of those key terms which everyone uses in talking about the arts, but which most of us would prefer not to have to define. It can mean so many different things in different contexts; far from embodying a unified set of doctrines, it represents an unsystematic and complicated skein of affinities between individual artists—a vast tangle of partial links and piecemeal influences.

If anything, its meaning is even harder to pin down than those of such comparably wide-ranging concepts as realism or romanticism, since unlike them it assigns a primary role to the idea of progress. That, after all, is where the use of “modernism” as a label puts the emphasis—not on a particular set of aesthetic or intellectual values, but on the virtue of change itself, and the desirability of moving ahead. “Behold, I make all things new.” But nothing stays modern forever, and what happens to such a movemen ...