We here a lot about the "middle class" in European-modeled societies, but most people lack a real understanding of what the middle class actually is.
The easiest and perhaps clearest method for defining the middle class is by exclusion: e.g., what is *not* the middle class?
We generally consider a modern society to have three classes: upper, middle, and lower.
The upper class is generally thought of as the "leisure class", or "the rich", or variations thereof. However, the primary characteristic of the upper class from a sociopolitical standpoint is this: their day-to-day livelihood and stability is ensured passively rather than actively. Their lives are not subject to the whims of other or even dependent on their own labor or activities.
To phrase it another way, those in the upper class generate incomes based on the efforts or workings of others: through investment, through ownership, through inheritance, etc. A person in the upper class need not do anything (or much) to maintain their lifestyle. Income is entirely passive. They can (and often do) choose to work in positions that interest them or provide some other benefit, but their core livelihood is ensured, for the most part, regardless of what they do. If a rich person decided to stay in bed all day (and a few have been rumored to do so), they would still be rich and continue to be rich.
The lower class - the poor - is largely marked by constant effort and vigilance to maintain living - not a lifestyle, but the actual necessities of day-to-day life. If a person in the lower class stops working, stops striving, stops putting out the effort - even for a short period - they often suffer major consequences with regards to their personal safety, security, health, or other basic needs.
The second characteristic of the lower class is high dependence on others: the state, employers, charity, etc. - as part of that struggle. Any interruption of support from those sources, all of which are outside the control of the individual, can also cause serious or disastrous personal consequences for a member of the lower class. That "support" can often be indirect; technically, a tenant is depending on the whims of a landlord in many cases in order to maintain their housing security.
To summarize: the upper class has (near-)total control over its basic needs and must expend little to any effort to maintain those needs, while the lower class has limited control over its basic needs and must expend nearly all of its effort to maintain them and/or live in a state of near-total dependence on forces outside of its control.
What, then, is the middle class?
The cheeky answer is "somewhere in the middle", but it's also the correct answer. The middle class must expend some, but not all, of its efforts towards its basic needs. It is less dependent on but not independent of others, a dependence most often in the form of employment.
From a simplified economic standpoint, we can consider the lower class as "those whose net worth is entirely equated with labor", the upper class as "those whose net worth is entirely separate from labor", and the middle class as "those whose net worth is a combination of labor and non-labor."
If we think about what we generally consider to be characteristics of the middle class, these definitions - and especially the economic distinctions - make sense. The middle class works for a living but can afford to take time off for vacation, meaning that day-to-day survival is not dependent on day-to-day labor. The middle class generally starts to accrue assets - car, home, retirement funds, maybe a small investment account; these assets are thus divorced from labor itself and can even constitute a small source of income/increased net worth that is labor-independent. The middle class also strives to ensure inheritance of some of those accrued assets, further enabling a separation of survival/net worth from labor.
This isn't to say that any of this is "right" in any ethical or moral sense; a large portion of leftist-progressive politics is based on how to separate labor from needs for the lower (and to a lesser extent middle) class, while right-conservative politics often looks to reduce dependence in the same groups. But understanding where the distinction actually lies may help shape these discussions.
For example, in the modern US, there is a lot of discussion about the "shrinking middle class"; this framing helps us understand exactly what that means. In short, it is an increase in the dependence that middle-class individuals must place on their labor or on sources outside their control to meet their basic needs; living "paycheck to paycheck" is a prime example of this, regardless of why someone is doing so. It's also a decrease in the likelihood of accruing real assets; for most families, a home is the primary asset, but fewer and fewer families can afford to own homes.
Another aspect of this shift is a change in dependency: historically, the middle class was dependent most on employers, in the form of healthcare, wages, pension, and other benefits. Many of those benefits have either disappeared entirely or are actively shifting out of the employer/employee relationship, with the result that those in the middle class are being left without that source of support. Again, we can discuss whether separating retirement plans from employment (via 401(k), IRAs, etc.) or healthcare from employment (via single-payer or a hybrid like the PPACA) is morally or ethically good in the long run, but it is inarguable that this relationship that in the past helped to build the middle class is deteriorating.
One clear example is simply the notion of company loyalty: It's now considered extremely odd to have only worked for one or two employers in one's career, whereas that used to be the standard. 40 years ago, an employee started at a company in their 20s and often retired from that company 30 years later. Today, the notion of loyalty from a company towards an employee (and, arguably as a consequence, from employee towards a company) has largely vanished; in some job sectors, working for the same organization for more than 24 months is considered a troubling sign on a resume.
As to why the middle class is considered so important in a society, that should be addressed elsewhere.
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