On the one hand, workers can't survive without selling their labor, so selling their labor is a way of making a living without choice, not free exchange; On the other hand, the essence of selling labor in the market is to alienate people into "things" as commodities, which is obviously the extreme manifestation of human alienation.

Therefore, "the slaves in Rome were chained, and the hired workers were tied to their masters by invisible threads", and "workers belonged to capital before selling themselves to capitalists".

The reason why capitalists can buy workers' labor and turn workers into hired workers lies in their ownership of the means of production. Under the capitalist system, capitalists possess the means of production and products of labor, while laborers have nothing but to make a living by selling their own labor.

There are two basic conditions for labor to become a commodity:

First, the laborer is a free man and can control his own labor force as his own commodity;

Second, workers have no other goods to sell, and they are free to have nothing, without any material conditions necessary to realize their own labor.