In this Oct. 23, 2014 photo, a downtown carpet business stands vacant in Danville, Ill. (AP Photo/Seth Perlman)

One of the questions that came up most often during Tuesday's State of the Union address -- and, it seems, in the intervening 24 hours -- is a simple one: What constitutes "middle class?" In our attempt to answer the most-Googled questions of the night, we touched on the answer.

Which is: It's vague.

We tend to think of middle class as a function of income. We think, for example, that someone making $5 million a year is not in the middle class. We think, too, that people who are poor -- say, someone who is living on federal assistance -- is not in the middle class, either. So it seems as though we should be able to assign a value to "middle class," as in, "middle class is for those who make an income between $25,000 and $80,000 a year."

The problem with that emerges immediately. In New York County, N.Y -- that is, Manhattan -- the median household income in 2013 was almost $69,659. In Tuscaloosa County, Ala., the median was $45,408. That's a function, among other things, of cost of living differences. But someone making $55,000 in Alabama is above average. Making that in New York, below. The status conferred by income is relative.

It's even relative to how much you make. We've pointed before to a 2013 poll from the Wall Street Journal and NBC News. Middle income, which seems like a good analogue for middle class, was at $50,000 to $75,000. But people who made more thought that the range was more likely to extend higher.

"No one views themselves at the bottom, they all view themselves in the middle class." said Wesley Yin, associate professor at the public policy school of the University of California, Los Angeles, and a former deputy assistant secretary with the Department of the Treasury in the Obama administration. "Even those at the bottom don't want policies to help the bottom. They want policies to help the middle class," he said.

So how does he -- a college professor and veteran of the federal government -- define "middle class?"

"I would look at it in terms of how our economy affects our opportunity," he said, not a dollar value. "People with very high income and a lot of opportunities and resources at their disposal, I wouldn't consider them middle class because of the way our economy is oriented to benefit those with high skills and assets."

For others, the lack of skills and assets put them at a disadvantage: no higher education, no savings, starting life in a low-income family. "It's hard to get out of that," Yin said. Everyone else? The middle class. People with some education, employed, people that could move up or down within the economy.

For the purposes of policy, that's more broad than is useful. "It's outcomes-based, rather than some measure of income," he said. "Technically, it's whatever you define it as." When you're doing economic analysis, "middle class" is relative to what you're looking at.

Yin gave the example of college affordability. The middle class might be considered those who are just able to afford college or emerge from it deep in debt. To Yin, that's the middle economic ground for that question -- not simply going to school and graduating or not being able to go at all.

When Obama sits down with his Council of Economic Advisers (for which Yin acted as a senior economist prior to going to Treasury), the group considers the composition of the population in terms of the metric that's being assessed, not according to some preset range of family incomes. At any given moment, you might be in the middle class, or you might not be.

And that's probably the best answer that can be given, for two reasons. First, because economics and social class is far more complicated than digits on a paycheck. And, second, because it is a category into which nearly every American sees him -- or herself fitting -- which is the sort of thing that political actors are loathe to try and discourage.