The key to understanding Marx is his class definition.1 A class is defined by the ownership of property. Such ownership vests a person with the power to exclude others from the property and to use it for personal purposes. In relation to property there are three great classes of society: the bourgeoisie (who own the means of production such as machinery and factory buildings, and whose source of income is profit), landowners (whose income is rent), and the proletariat (who own their labor and sell it for a wage).
Class thus is determined by property, not by income or status. These are determined by distribution and consumption, which itself ultimately reflects the production and power relations of classes. The social conditions of bourgeoisie production are defined by bourgeois property. Class is therefore a theoretical and formal relationship among individuals.
The force transforming latent class membership into a struggle of classes is class interest. Out of similar class situations, individuals come to act similarly. They develop a mutual dependence, a community, a shared interest interrelated with a common income of profit or of wages. From this common interest classes are formed, and for Marx, individuals form classes to the extent that their interests engage them in a struggle with the opposite class.
At first, the interests associated with land ownership and rent are different from those of the bourgeoisie. But as society matures, capital (i.e., the property of...