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, p.405

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

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

In so far as each single transaction invariably conforms to the laws of the exchange of commodities, the capitalist buying labor-power, the laborer selling it, and we will assume at its real value; in so far as all of this is true, it is evident that the laws of appropriation or of private property, laws that are based on the production and circulation of commodities, become by their own inner and inexorable dialectic changed into their very opposite. The exchange of equivalents, the original operation with which we started, has now become turned round in such a way that there is only an apparent exchange. This is owing to the fact, first, that the capital which is exchanged for labor-power is itself but a portion of the product of other’s labor appropriated without an equivalent; and, secondly, that this capital must not only be replaced by its producer, but replaced together with an added surplus. The relation of exchange subsisting between capitalist and laborer becomes a mere semblance appertaining to the process of circulation, a mere form, foreign to the real nature of the transaction, and only mystifying it. The ever-repeated purchase and sale of labor-power is now the mere form; what really takes place is this – the capitalist again and again appropriates, without equivalent, a portion of the previously materialized labor of others, and exchanges it for a greater quantity of living labor. At first the rights of property seemed to us to be based on a man’s own labor. [...] Now, however, property turns out to be the right, on the part of the capitalist, to appropriate the unpaid labor of others or its products, and to be the impossibility, on the part of the laborer, of appropriating his own product. The separation of property from labor has become the necessary consequence of a law that apparently originated in their identity. Marx, Capital, Volume 1 (Progress Publishers, 1887), p.412-413

However much the capitalist mode of appropriation may seem to fly in the face of the original laws of commodity production, it nevertheless arises, not from a violation, but, on the contrary, from the application of these laws. Let us make this clear once more by briefly reviewing the consecutive phases of motion whose culminating point is capitalist accumulation. We saw, in the first place, that the original conversion of a sum of values into capital was achieved in complete accordance with the laws of exchange. One party to the contract sells his labor-power, the other buys it. The former receives the value of his commodity, whose use value – labor – is thereby alienated to the buyer. Means of production which already belong to the latter are then transformed by him, with the aid of labor equally belonging to him, into a new product which is likewise lawfully his. The value of the product includes: first, the value of the used-up means of production. Useful labor cannot consume these means of production without transferring their value to the new product, but, to be saleable, labor-power must be capable of supplying useful labor in the branch of industry in which it is to be employed. The value of the new product further includes: the equivalent of the value of labor-power together with a surplus-value. This is so because the value of labor-power – sold for a definite length of time, say a day, a week etc – is less than the value created by its use during that time. But the worker has received payment for the exchange-value of his labor-power and by so doing has alienated its use value – this being the case in every sale and purchase. The fact that this particular commodity, labor-power, possesses the peculiar use value of supplying labor, and therefore, of creating value, cannot affect the general law of commodity production. If, therefore, the magnitude of the value advanced in wages is not merely found again in the product, but is found there augmented by a surplus-value, this is not because the seller has been defrauded, for he has really received the value of his commodity; it is due solely to the fact that this commodity has been used up by the buyer. The law of exchange requires equality only between the exchange-values of the commodities given in exchange for one another. From the very outset, it presupposes even a difference between their use-values and it has nothing whatever to do with their consumption, which only begins after the deal is closed and executed. Thus the original conversion of money into capital is achieved in the most exact accordance with the economic laws of commodity production and with the right of property derived from them. Nevertheless, the result is: 1. That the product belongs to the capitalist and not to the worker 2. That the value of this product includes, besides the value of the capital originally advanced, a surplus-value which costs the worker labor but the capitalist nothing, and which nonetheless becomes the legitimate property of the capitalist 3. That the worker has retained his labor-power and can sell it anew if he can find a buyer. Simple reproduction is only the periodical repetition of this first operation; each time money is converted afresh into capital. Thus the law is not broken; on the contrary, it is merely enabled to operate continuously. [...] Nor does it matter if simple reproduction is replaced by reproduction on an extended scale, by accumulation. In the former case the capitalist squanders the whole surplus-value in dissipation, in the latter he demonstrates his bourgeois virtue by consuming only a portion of it and converting the rest into money. Marx, Capital, Volume 1 (Progress Publishers, 1887), p.413-414

If the product of labour does not belong to the worker, if it confronts him as an alien power, then this can only be because it belongs to some other man than the worker. If the worker's activity is a torment to him, to another it must give satisfaction and pleasure. Not the gods, not nature, but only man himself can be this alien power over man. We must bear in mind the previous proposition that man's relation to himself only becomes for him objective and actual through his relation to the other man. Thus, if the product of his labour, his labour objectified, is for him an alien, hostile, powerful object independent of him, then his position towards it is such that someone else is master of this object, someone who is alien, hostile, powerful, and independent of him. If he treats his own activity as an unfree activity, then he treats it as an activity performed in the service, under the dominion, the coercion, and the yoke of another man. […] through estranged labour man not only creates his relationship to the object and to the act of production as to powers" that are alien and hostile to him; he also creates the relationship in which other men stand to his production and to his product, and the relationship in which he stands to these other men. Just as he creates his own production as the loss of his reality, as his punishment; his own product as a loss, as a product not belonging to him; so he creates the domination of the person who does not produce over production and over the product. Just as he estranges his own activity from himself, so he confers upon the stranger an activity which is not his own. Marx, Economic and Philosophic Manuscripts, Marx Engels Collected Works Volume 3 (International Publishers), p.278-279