We have assumed that labor-power is bought and sold at its value, as determined by the labor-time necessary to produce it. However, the amount of labor needed to provide subsistence does not always equal the length of the work-day. The time in excess of the necessary labor time is surplus labor. Thus, the working day is a variable quantity, which changes according to the amount of surplus labor. However, it can only vary within limits. There is no real minimum limit; there must be some surplus labor because of the nature of the capitalist system, and it can approach but not reach zero. The maximum is constrained by physical limitations and moral constraints, such as the need to fulfill other obligations.
The capitalist has a very particular view on this matter. "As a capitalist, he is only capital personified. His soul is the soul of capital." Capital's drive is to create surplus-value, and make the means of production absorb as much surplus labor as possible. If a worker uses his disposable time for himself, he is effectively robbing the capitalist, because the capitalist lives on surplus labor. Thus, the capitalist tries to get the maximum possible benefit from the worker's use-value.
However, the worker has his own view about how much he should work. His labor-power is different from other commodities, because it creates value. From the worker's perspective, the capitalist's demands reflect an excess expenditure of labor-power. For example, a capitalist could potentially use so much labor-power in a day that it would take three days to restore it. "Using my labor and despoiling it are quite different things." The worker argues that the capitalist can't use three days worth of his labor power and only pay him for one. He demands the value of his commodity.
Based on the principles of commodity exchange, both sides have equally valid rights in this case. Here, force will be the solution, and the history of capitalist production reflects such tension between the capitalists and the workers.
An important theme in Marx's work is class tension. According to Marx, all of history has been defined by class conflict. Modern times are no different in this regard, and are defined by tension between the capitalist and the worker. Marx describes one source of this tension in this chapter, as he mentions again the asymmetry between the use-value and exchange-value of labor-power (already discussed in Chapter 7). In this class conflict, the capitalists are the stronger class. This allows them to exert more force and define what workers will be paid. However, the fact that they are the stronger class does not simply give capitalists more bargaining power. Rather, social institutions such as property laws are defined to support the capitalists' needs. The mode of production reflects the economic system of capitalism. It will continue to do so, and continue to favor the capitalists, until it self-destructs.
It is important to realize that the capitalists cannot behave differently; there will always be tension between them and the workers. The very essence of a capitalist is his desire to gain surplus-value. The only way to do so is to exploit workers by failing to pay workers for the full value of what they produce. In order to survive, the capitalist must exploit. Thus, the tension between workers and capitalists is structural. The capitalist system requires exploitation. Measures to ease workers' hardships, such as a minimum wage or welfare are simply band-aids; they cannot change what a capitalist is.