The main problem with this approach is obvious: The same income buys a vastly different quality of life in different parts of the country. That is to say, $60,000 goes much further in Missoula, Montana, than in Brooklyn, New York.

There’s another problem: Class has always been about more than earnings. For many decades, social scientists and historians have debated how jobs, education, politics, consumption, and, yes, income come together with values, habits, geography, and social status to create class in America. In other words, the question is not who but what makes the middle class. Today there can be no pretending that middle-class status is anchored by a single economic reality. Instead, it is primarily an aspiration.

Being middle class means striving for the stability and respectability that older generations achieved by holding down steady jobs, owning a home, and raising upright kids who could take their place. These benchmarks are no longer simple to attain. Instead, middle-class desires are marred by an insecurity historically associated with the American working class. Definitions should reflect that.

In his 1951 landmark book, White Collar, the sociologist C. Wright Mills offered a way to study the middle class that remains useful today, despite vastly changed circumstances. He did not seek merely to delineate the middle class, but to explain what it is like—socially and psychologically—to live in the middle of the class structure.

Since the 1830s, clerks and small-scale entrepreneurs had been carving out a space between factory workers and factory owners in northern cities. Men worked with their head, not their hands. They spent their days bent over account books in offices while their wife stayed behind to make sure that the home was clean, the cupboard stocked, and the children tended to.

By the mid-1900s, this in-between class worked in large, impersonal bureaucratic organizations that changed little. They managed people and manipulated numbers and words, keeping corporate offices and government bureaus humming with activity.

Read: The decline of social mobility in America

Unlike laborers below them, white-collar workers were unlikely to send a piece of their paycheck to local AFL-CIO chapters. They enjoyed higher, more consistent pay, and were willing to accept company goals as their own. Mills’s colleague William H. Whyte found that these office workers identified so completely with their corporate or government job that he named a new social type for them: “the organization man.”

Organization men were attached to their firm— they were psychologically “dependent.” Beyond their personal commitments, moreover, these mid-rung workers were dependent in another crucial way: They did not make decisions about the shape of their job. Owners and high-level executives controlled and spelled-out white-collar responsibilities, from pushing paper to smiling at potential customers.