In the first of his Economic Manuscripts, published in 1844, Marx described the process of alienation as follows:
The worker becomes all the poorer the more wealth he produces, the more his production increases in power and size. The worker becomes an ever cheaper commodity the more commodities he creates. The devaluation of the world of men is in direct proportion to the increasing value of the world of things. Labor produces not only commodities; it produces itself and the worker as a commodity—and this at the same rate at which it produces commodities in general.
This was one of Marx's most important insights and critiques of industrial capitalism. Under a system of wage labor, workers became economic units in the same sense the things they produced were. A worker who labored in a sweatshop to produce shoes, for example, sold his labor for a price. This was dehumanizing inasmuch as it commodified human beings, alienating them from their fully-realized selves.
Marx, of course, thought in economic terms, and he also pointed out that workers were being increasingly alienated from the value of the things they produced. The things they produced were sold for a price, but the workers themselves received wages that represented only a fraction of the value of these things. The bourgeoisie, who owned the means of production, received far more of the proceeds than the people who actually labored to produce them. Marx recognized that the more efficiently things were produced, the more value was generated and the more alienated workers were from the value of their labor. "[T]he worker," Marx wrote, "is related to the product of labor as to an alien object."
Moreover, the more efficient labor became under the new industrial system, the less "affirming" it was. People who worked in exhausting, repetitive work were not fulfilling themselves through their work in the same way that craftsmen who applied their expertise to their jobs were. Marx compared this kind of work to that performed by draft animals. Fundamentally, it alienated workers from their humanity. The fact that workers would become increasingly alienated from their work, the value of their labor, and from their own humanity as the bourgeoisie maximized efficiency and profits was one reason Marx foretold the collapse of capitalism.