More often than not, the history of art is presented in shapely chunks, each “ism” nicely circumscribed and clearly defined, in a neat ideological and chronological progression. But reality is far messier than this orderly structure suggests, the phenomena of period and national style, not to mention the Zeitgeist, notwithstanding. Individuals and even whole “isms” rarely fit tidily into time charts. There’s all that in-between territory and all those people with radically differing ideas about art whose lifetimes and careers overlap disconcertingly. Corot, for example, born in 1796, was two years older than Delacroix but lived until 1875, long enough to exhibit at the same time as Monet, forty-four years his junior. Monet lived until 1926, almost twenty years after Picasso painted Les Demoiselles d’Avignon and two years after the Surrealists published their first Manifesto.

The Dutch historian Johan Huizinga, in his classic study The Waning of the Middle Ages, described the transition between the medieval period and the Renaissance as an incoming tide, a series of waves inexorably moving inland, some breaking higher, some lower. You could call the low-water mark the end of the Middle Ages and the high-water mark the start of the Renaissance, but what about that stretch of wet beach in between? With its record of unsystematic advances and retreats, it could be the most thought-provoking area to explore. The current exhibition at the Metropolitan, “Origins of Impressionism,” is an effort to examine just such a zone, the region below the