“The history of all hitherto existing society is the history of class struggles.” So opens the first section of The Communist Manifesto. For Marx, classes are the main actors of history, and the conflict between them is its engine.

What a class is

For Marx, a class is defined by its relationship to the means of production, not by income, education or lifestyle. The key question is: do you own the things that produce wealth (factories, land, machines, capital), or do you have to sell your ability to work to someone who does?

In capitalism this gives two main classes:

• The bourgeoisie: owners of the means of production, who live mainly on profit, rent and interest.
• The proletariat: workers who own no means of production and must sell their labor for wages to live.

Marx recognized other groups too: the petty bourgeoisie (small shopkeepers, artisans and farmers who own a little and work it themselves), landlords, and the lumpenproletariat (people pushed outside regular work altogether). He expected competition to squeeze many of the petty bourgeoisie into the working class over time.

Why classes conflict

Marx argued that class interests clash in a structural way, regardless of whether individual employers are kind or cruel. Owners need to keep costs low and output high to survive competition; workers want higher wages, shorter hours and safer conditions. Every gain for one side is, in an important sense, a cost to the other. Strikes, lockouts, labor laws and political parties are all forms this struggle takes.

A class in itself and for itself

Marx drew a distinction that later Marxists summarized as a class “in itself” and a class “for itself”. A class in itself simply shares a common position, such as millions of workers who each sell their labor. A class for itself is aware of its shared interests and organizes to pursue them: unions, workers' parties, movements. Marx thought capitalism, by gathering workers into large factories and cities, would help turn the first into the second.

Ideology

If workers have strong shared interests, why don't they always act on them? Marx's answer centers on ideology. In The German Ideology, he and Engels wrote that “the ideas of the ruling class are in every epoch the ruling ideas.” The class that controls material production also has outsized influence over the production of ideas: schools, newspapers, churches and, today, media.

Ideology in this sense is not simple lying. It is a set of ideas that present the interests of one class as natural, universal or inevitable. The belief that market outcomes are simply what everyone deserves, or that existing property arrangements are the only realistic ones, would be examples a Marxist would point to. Engels later used the phrase “false consciousness” for thinking that hides a person's real situation from them.

Religion

Marx's famous line that religion is “the opium of the people” is often quoted alone. In context (1843) he also called religion “the sigh of the oppressed creature, the heart of a heartless world.” His point was that religion offers real comfort in real suffering, while also, in his view, directing attention away from changing the conditions that cause the suffering.

The state

The Manifesto describes the modern state as “a committee for managing the common affairs of the whole bourgeoisie.” Later Marxists argued over how literally to take this. Some saw the state as a direct tool of the capitalist class; others saw it as partly independent, able to pass reforms that workers won through struggle.

Worked example: class in a modern economy

Take three people. A software engineer earning a high salary, a cashier earning minimum wage, and a woman who owns three rental houses and does not otherwise work.

By income, the engineer and the landlord might sit near each other, far above the cashier. By Marx's definition, the engineer and the cashier are in the same basic class: both must sell their labor to an employer to live. The landlord lives on income from property she owns, which puts her on the other side of the relationship.

Later Marxists recognized that this gets complicated. Managers sell their labor but direct other workers on the owners' behalf. Many workers own shares through pension funds. The sociologist Erik Olin Wright called such people occupants of contradictory class locations. Critics argue these complications show that a two-class model no longer fits modern societies; Marxists reply that the core relationship, owning versus selling labor, still shapes most people's lives.

A useful exercise: for any political debate about wages, taxes or housing, ask which positions people take, and whether they line up with their relationship to property more than with their income alone.