All trends become clearer with time. Looking at art even 15 years out, “you can see the patterns a little better,” says Melissa Ho, assistant curator at the Hirshhorn Museum. “There are larger, deeper trends that have to do with how we are living in the world and how we are experiencing it.”

So what exactly is modern art? The question, she says, is less answerable than endlessly discussable.

Technically, says Ho, modern art is “the cultural expression of the historical moment of modernity.” But how to unpack that statement is contested. One way of defining modern art, or anything really, is describing what it is not. Traditional academic painting and sculpture dominated the 17th, 18th and 19th centuries. “It was about perfect, seamless technique and using that perfect, seamless technique to execute very well-established subject matter,” says Ho. There was a hierarchy of genres, from history paintings to portraiture to still lifes and landscapes, and very strict notions of beauty. “Part of the triumph of modernism is overturning academic values,” she says.

In somewhat of a backlash to traditional academic art, modern art is about personal expression. Though it was not always the case historically, explains Ho, “now, it seems almost natural that the way you think of works of art are as an expression of an individual vision.” Modernism spans a huge variety of artists and kinds of art. But the values behind the pieces are much the same. “With modern art, there is this new emphasis put on the value of being original and doing something innovative,” says Ho.

Edouard Manet and the Impressionists were considered modern, in part, because they were depicting scenes of modern life. The Industrial Revolution brought droves of people to the cities, and new forms of leisure sprung up in urban life. Inside the Hirshhorn’s galleries, Ho points out Thomas Hart Benton’s People of Chilmark, a painting of a mass of tangled men and women, slightly reminiscent of a classical Michelangelo or Théodore Géricault’s famous Raft of the Medusa, except that it is a contemporary beach scene, inspired by the Massachusetts town where Benton summered. Ringside Seats, a painting of a boxing match by George Bellows, hangs nearby, as do three paintings by Edward Hopper, one titled First Row Orchestra of theatergoers waiting for the curtains to be drawn.

In Renaissance art, a high premium was put on imitating nature. “Then, once that was chipped away at, abstraction is allowed to flourish,” says Ho. Works like Benton’s and Hopper’s are a combination of observation and invention. Cubists, in the early 1900s, started playing with space and shape in a way that warped the traditional pictorial view.

Art historians often use the word “autonomous” to describe modern art. “The vernacular would be ‘art for art’s sake,’” explains Ho. “It doesn’t have to exist for any kind of utility value other than its own existential reason for being.” So, assessing modern art is a different beast. Rather than asking, as one might with a history painting, about narrative—Who is the main character? And what is the action?—assessing a painting, say, by Piet Mondrian, becomes more about composition. “It is about the compositional tension,” says Ho, “the formal balance between color and line and volume on one hand, but also just the extreme purity of and rigor of it.”

According to Ho, some say that modernism reaches its peak with Abstract Expressionism in America during the World War II era. Each artist of the movement tried to express his individual genius and style, particularly through touch. “So you get Jackson Pollock with his dripping and throwing paint,” says Ho. “You get Mark Rothko with his very luminous, thinly painted fields of color.” And, unlike the invisible brushwork in heavily glazed academic paintings, the strokes in paintings by Willem de Kooning are loose and sometimes thick. “You really can feel how it was made,” says Ho.

Shortly after World War II, however, the ideas driving art again began to change. Postmodernism pulls away from the modern focus on originality, and the work is deliberately impersonal. “You see a lot of work that uses mechanical or quasi-mechanical means or deskilled means,” says Ho. Andy Warhol, for example, uses silk screen, in essence removing his direct touch, and chooses subjects that play off of the idea of mass production. While modern artists such as Mark Rothko and Barnett Newman made color choices that were meant to connect with the viewer emotionally, postmodern artists like Robert Rauschenberg introduce chance to the process. Rauschenburg, says Ho, was known to buy paint in unmarked cans at the hardware store.

“Postmodernism is associated with the deconstruction of the idea, ‘I am the artistic genius, and you need me,’ ” says Ho. Artists such as Sol LeWitt and Lawrence Weiner, with works in the Hirshhorn, shirk authorship even more. Weiner’s piece titled “A RUBBER BALL THROWN ON THE SEA, Cat. No. 146,” for example, is displayed at the museum in large, blue, sans-serif lettering. But Weiner was open to the seven words being reproduced in any color, size or font. “We could have taken a marker and written it on the wall,” says Ho. In other words, Weiner considered his role as artist to be more about conception than production. Likewise, some of LeWitt’s drawings from the late 1960s are basically drawings by instruction. He provides instructions but anyone, in theory, can execute them. “In this post-war generation, there is this trend, in a way, toward democratizing art,” says Ho. “Like the Sol LeWitt drawing, it is this opinion that anybody can make art.”

Labels like “modern” and “postmodern,” and trying to pinpoint start and end dates for each period, sometimes irk art historians and curators. “I have heard all kinds of theories,” says Ho. “I think the truth is that modernity didn’t happen at a particular date. It was this gradual transformation that happened over a couple hundred of years.” Of course, the two times that, for practical reasons, dates need to be set are when teaching art history courses and organizing museums. In Ho’s experience, modern art typically starts around the 1860s, while the postmodern period takes root at the end of the 1950s.

The term “contemporary” is not attached to a historical period, as are modern and postmodern, but instead simply describes art “of our moment.” At this point, though, work dating back to about 1970 is often considered contemporary. The inevitable problem with this is that it makes for an ever-expanding body of contemporary work for which professors and curators are responsible. “You just have to keep an eye on how these things are going,” advises Ho. “I think they are going to get redefined.”