The breakdown of history into arbitrary, discrete segments called decades or centuries seems silly and misleading. The Sixties didn’t start on January 1st, 1960 and end on December 31st, 1969, after all. Everyone knows they started when Ed Sullivan introduced the Beatles on February 9th, 1964 and ended when the Hells Angels sacrificed a young, black man at the Rolling Stones’ free concert in Altamont on December 6th, 1969. Though some argue they started when Sputnik flew on October 4th, 1957 and ended when man last walked the moon on December 14th, 1972. Each of these delineations may be ridiculous. Yet we know what we’re talking about when we talk about the Sixties. Or the Eighties, or the Thirties. Each of us has a clear mental picture, informed by a lifetime of schooling and media consumption, of what each era signifies.

But maybe it’s not so arbitrary. Maybe we don’t, in hindsight, read a pattern in a few signal events that happen to have occurred in the same decade, or century, and interpret that pattern to “mean” something, and attribute that meaning to the entire period. Something else is at play. We are conscious of these periods as we live them, and to some degree we behave—think, believe, act—in accordance to what we believe to be the prevailing spirit of the time. In other words, people did things in the Sixties—drop acid, listen to rock music, protest against the war—not just because that’s where the currents of history had carried them but because they were conscious that they were living in the Sixties and that doing those things, and feeling the way they felt, is what was expected of them as “citizens” of the decade. And when it became the Seventies—on January 1st, 1970, or at least within a few weeks of then—people started to do the sorts of things we now identify with the Seventies—snort coke, listen to disco, swap spouses—because they knew it was the Seventies.

President Obama will be remembered for having dragged the United States—much of it kicking and screaming—into the 21st century.