Karl Marx's pamphlet Wage Labour and Capital first appeared as a series of articles in Neue Rheinische Zeitung, the newspaper that Marx edited during the 1848-9 revolution that swept Germany and Europe.
The articles were based lectures that Marx had given to German workers in Brussels in 1847.
Marx's aim in the pamphlet is to set out and explain "the economic conditions which form the material basis of the present struggles between classes."
The bourgeoisie and the proletariat initially fought side by side against the old feudal order in 1848. But this apparent harmony of interests between these two classes in the revolution increasingly gave way to open conflict. This in turn pushed the bourgeoisie to side with the counter revolution against the growing threat from below, which threatened the property of the capitalists as much as the old feudal landowners.
Wage Labour and Capital was thus directed at workers as part of Marx's fight for the political independence of the working class in the revolution.
Marx starts with the wages which are paid for by a capitalist in return for the worker's capacity to work, their labour power. Labour power under capitalism is a commodity, to be bought and sold just like any other. But labour power is distinctive compared with to every other commodity in one very important way. Uniquely, when it is put to use it performs more labour than is required to produce it.
The value of labour power depends, like all other commodities, on the amount of labour needed to produce it. If workers are to provide labour power to the capitalists they require food, housing, clothing and so on. Their wage has to cover the cost of these things - it has to match the amount of social labour needed to produce them. Wages also have to cover the cost of bringing up the next generation of workers, the workers' children who are the future providers of labour power to the capitalists.
These costs - the production and reproduction of labour power - are what ultimately determine the "price" of labour power, the worker's wage.
But the labour that workers perform can be greater than the amount of labour required to meet these costs. It might, for example, take someone only 3 hours a day to produce enough to cover the cost of replenishing their labour power. But they may be employed to work 8, 9 or ten hours day. The extra labour is pocketed by the capitalist. It is this surplus value which is the basis of the capitalists' profits.
One reason that it can appear that there is no inherent conflict of interest between workers and capitalists is that this form of exploitation is hidden beneath the wage contract.
The relationship between employer and worker has the outward appearance of one between equals. The employer agrees to provide a wage in return for the worker providing his or her labour. Unlike a slave who is the property of the master, or the feudal serf who is tied to their land and lord, the worker is not compelled by force to work. He or she is free in this sense and the sale of labour power in exchange for a wage appears to be a voluntary act.
Yet this formal equality hides the deeper underlying inequality between the worker and capitalist. The capitalist is in possession of means of making a livelihood - the factories, mines, offices and so on - and the worker is compelled to work for them if the want to avoid either a life on miserable benefits or outright starvation.
Workers are "free" in the sense that they do not have to work for any individual capitalist, but they cannot avoid working for the capitalist class as a whole.
"The worker leaves the capitalist, to whom he has sold himself, as often as he chooses, and the capitalist discharges him as often as he sees fit, as soon as he no longer gets any use, or not the required use, out of him. But the worker, whose only source of income is the sale of his labour-power, cannot leave the whole class of buyers, i.e., the capitalist class, unless he gives up his own existence. He does not belong to this or that capitalist, but to the capitalist class; and it is for him to find his man - i.e., to find a buyer in this capitalist class."
And each time the worker sells his labour power to the capitalist, the resulting surplus value that is extracted from him or her can be used by the capitalist to accumulate further means of production - indeed the capitalist is compelled by competition with other capitalists to re-invest profits on pain of extinction.
As the capitalists accumulates further means of production in his or her hands, so too does it act to increase the capitalist's ability to blackmail workers into working on unequal terms.
"Does a worker in a cotton factory produce only cotton? No. He produces capital. He produces values which serve anew to command his work and to create by means of it new values."
So capital is not simply material means of production but accumulated past labour, which dominates the lives of living labour. So, as Marx puts it, "the interests of capitals and the interests of wage labour are diametrically opposed to each other." The class struggle must be fought out on this basis, and this basis alone.