The connection between [the productive] forces and [the social] relations is an illuminating one. Among other things, it allows us to recognize that you can only have certain social relations if the productive forces have evolved to a certain extent. If some people are to live a lot more comfortably than others, you need to produce a sizable economic surplus; and this is possible only at a certain point of productive development. You cannot sustain an immense royal court complete with minstrels, pages, jesters, and chamberlains if everyone has to herd goats or grub for plants all the time just to survive.
The class struggle is essentially a struggle over the surplus, and as such is likely to continue as long as there is not a sufficiency for all. Class comes about whenever material production is so organized as to compel individuals to transfer their surplus labor to others in order to survive. When there is little or no surplus, as in so-called primitive communism, everyone has to work, nobody can live off the toil of others, and so there can be no classes. Later, there is enough of a surplus to fund classes like feudal lords, who live by the labor of their underlings. Only with capitalism can enough surplus be generated for the abolition of scarcity, and thus of social classes, to become possible. But only socialism can put this into practice.
— Terry Eagleton