Whatever the form of the process of production in a society, it must be a continuous process, must continue to go periodically through the same phases. A society can no more cease to produce than it can cease to consume. When viewed, therefore, as a connected whole, and as flowing on with incessant renewal, every social process of production is, at the same time, a process of reproduction. The conditions of production are also those of reproduction. No society can go on producing, in other words, no society can reproduce, unless it constantly reconverts a part of its products into means of production, or elements of fresh products. All other circumstances remaining the same, the only mode by which it can reproduce its wealth, and maintain it at one level, is by replacing the means of production – i.e., the instruments of labour, the raw material, and the auxiliary substances consumed in the course of the year – by an equal quantity of the same kind of articles; these must be separated from the mass of the yearly products, and thrown afresh into the process of production. Hence, a definite portion of each year’s product belongs to the domain of production. Destined for productive consumption from the very first, this portion exists, for the most part, in the shape of articles totally unfitted for individual consumption. Marx, Capital, Volume 1 (Progress Publishers, 1887), p.401

Variable capital is therefore only a particular historical form of appearance of the fund for providing the necessaries of life, or the labour-fund which the labourer requires for the maintenance of himself and family, and which, whatever be the system of social production, he must himself produce and reproduce. If the labour-fund constantly flows to him in the form of money that pays for his labour, it is because the product he has created moves constantly away from him in the form of capital. But all this does not alter the fact, that it is the labourer’s own labour, realised in a product, which is advanced to him by the capitalist. Let us take a peasant liable to do compulsory service for his lord. He works on his own land, with his own means of production, for, say, 3 days a week. The 3 other days he does forced work on the lord’s domain. He constantly reproduces his own labour-fund, which never, in his case, takes the form of a money payment for his labour, advanced by another person. But in return, his unpaid forced labour for the lord, on its side, never acquires the character of voluntary paid labour. If one fine morning the lord appropriates to himself the land, the cattle, the seed, in a word, the, means of production of this peasant, the latter will thenceforth be obliged to sell his labour-power to the lord. He will, ceteris paribus, labour 6 days a week as before, 3 for himself, 3 for his lord, who thenceforth becomes a wages-paying capitalist. As before, he will use up the means of production as means of production, and transfer their value to the product. As before, a definite portion of the product will be devoted to reproduction. But from the moment that the forced labour is changed into wage labour, from that moment the labour-fund, which the peasant himself continues as before to produce and reproduce, takes the form of a capital advanced in the form of wages by the lord. The bourgeois economist whose narrow mind is unable to separate the form of appearance from the thing that appears, shuts his eyes to the fact, that it is but here and there on the face of the earth, that even nowadays the labour fund crops up in the form of capital. Marx, Capital, Volume 1 (Progress Publishers, 1887), p.402

Apart then from all accumulation, the mere continuity of the process of production, in other words simple reproduction, sooner or later, and of necessity, converts every capital into accumulated capital, or capitalized surplus-value. Even if that capital was originally acquired by the personal labor of its employer, it sooner or later becomes value appropriated without an equivalent, the unpaid labor of others materialized either in money or in some other object. We saw that in order to convert money into capital something more is required than the production and circulation of commodities. We saw that on the one side the possessor of value or money, on the other, the possessor of the value-creating substance; on the one side, the possessor of the means of production and subsistence, on the other, the possessor of nothing but labor-power, must confront one another as buyer and seller. The separation of labor from its product, of subjective labor-power from the objective conditions of labor, was therefore the real foundation in fact, and the starting point of capitalist production. But that which at first was but a starting point, becomes, by the mere continuity of the process, by simple reproduction, the peculiar result, constantly renewed and perpetuated, of capitalist production. On the one hand, the process of production incessantly converts material wealth into capital, into means of creating more wealth and means of enjoyment for the capitalist. On the other hand, the laborer, on quitting the process, is what he was on entering it, a source of wealth, but devoid of all means of making that wealth his own. Since, before entering on the process, his own labor has already been alienated from himself by the sale of his labor-power, has been appropriated by the capitalist and incorporated with capital, it must, during the process, be realized in a product that does not belong to him. Since the process of production is also the process by which the capitalist consumes labor-power, the product of the laborer is incessantly converted, not only into commodities, but into capital, into value that sucks-up the value creating power, into means of subsistence that buy the person of the laborer, into means of production that command the producers. The laborer therefore constantly produces material, objective wealth, but in the form of capital, of an alien power that dominates and exploits him; and the capitalist as constantly produces labor-power, but in the form of a subjective source of wealth, separated from the objects in and by which it can alone be realized; in short he produces the laborer, but as a wage laborer. This incessant reproduction, the perpetuation of the laborer, is the sine qua non of capitalist production. […] Capitalist production, therefore, of itself reproduces the separation between labor-power and the means of labor. It thereby reproduces and perpetuates the condition for exploiting the laborer. It incessantly forces him to sell his labor-power in order to live and enables the capitalist to purchase labor-power in order that he may enrich himself. It is no longer a mere accident, that capitalist and laborer confront each other in the market as buyer and seller. It is the process itself that incessantly hurls back the laborer on to the market as a vendor of his labor-power and that incessantly converts his own product into a means by which another man can purchase him. In reality, the laborer belongs to capital before he has sold himself to capital. His economic bondage is both brought about and concealed by the periodic sale of himself, by his change of masters, and by the oscillations in the market-price of labor-power. Capitalist production, therefore, under its aspect of a continuous connected process, of a process of reproduction, produces not only commodities, not only surplus-value, but it also produces and reproduces the capitalist relation; on the one side the capitalist, on the other the wage laborer. Marx, Capital, Volume 1 (Progress Publishers, 1887), p.403, 407

We saw that on the one side the possessor of value or money, on the other, the possessor of the value-creating substance; on the one side, the possessor of the means of production and subsistence, on the other, the possessor of nothing but labor-power, must confront one another as buyer and seller. The separation of labor from its product, of subjective labor-power from the objective conditions of labor, was therefore the real foundation in fact, and the starting point of capitalist production. But that which at first was but a starting point, becomes, by the mere continuity of the process, by simple reproduction, the peculiar result, constantly renewed and perpetuated, of capitalist production. On the one hand, the process of production incessantly converts material wealth into capital, into means of creating more wealth and means of enjoyment for the capitalist. On the other hand, the laborer, on quitting the process, is what he was on entering it, a source of wealth, but devoid of all means of making that wealth his own. Since, before entering on the process, his own labor has already been alienated from himself by the sale of his labor-power, has been appropriated by the capitalist and incorporated with capital, it must, during the process, be realized in a product that does not belong to him. Since the process of production is also the process by which the capitalist consumes labor-power, the product of the laborer is incessantly converted, not only into commodities, but into capital, into value that sucks-up the value creating power, into means of subsistence that buy the person of the laborer, into means of production that command the producers. The laborer therefore constantly produces material, objective wealth, but in the form of capital, of an alien power that dominates and exploits him; and the capitalist as constantly produces labor-power, but in the form of a subjective source of wealth, separated from the objects in and by which it can alone be realized; in short he produces the laborer, but as a wage laborer. This incessant reproduction, the perpetuation of the laborer, is the sine qua non of capitalist production. Marx, Capital, Volume 1 (Progress Publishers, 1887), p.403

The Roman slave was held by fetters: the wage labourer is bound to his owner by invisible threads. The appearance of independence is kept up by means of a constant change of employers, and by the fictio juris of a contract. Marx, Capital, Volume 1 (Progress Publishers, 1887), p.405