We proceed from an actual economic fact. The worker becomes all the poorer the more wealth he produces, the more his production increases in power and size. The worker becomes an ever cheaper commodity the more commodities he creates. The devaluation of the world of men is in direct proportion to the increasing value of the world of things. Labour produces not only commodities: it produces itself and the worker as a commodity— and this at the same rate at which it produces commodities in general. This fact expresses merely that the object which labour produces—labour's product—confronts it as something alien, as a power independent of the producer. The product of labour is labour which has been embodied in an object, which has become material: it is the objectification of labour. Labour's realisation is its objectification. Under these economic conditions this realisation of labour appears as loss of realisation for the workers; objectification as loss of the object and bondage to it; appropriation as estrangement, as alienation. Marx, Economic and Philosophic Manuscripts, Marx Engels Collected Works Volume 3 (International Publishers), p.271-272

Till now we have been considering the estrangement; the alienation of the worker only in one of its aspects, i.e., the worker's relationship to the products of his labour. But the estrangement is manifested not only in the result but in the act of production, within the producing activity itself. How could the worker come to face the product of his activity as a stranger, were it not that in the very act of production he was estranging himself from himself? The product is after all but the summary of the activity, of production. If then the product of labour is alienation, production itself must be active alienation, the alienation of activity, the activity of alienation. In the estrangement of the object of labour is merely summarised the estrangement, the alienation, in the activity of labour itself. What, then, constitutes the alienation of labour? First, the fact that labour is external to the worker, i.e., it does not belong to his intrinsic nature; that in his work, therefore, he does not affirm himself but denies himself, does not feel content but unhappy, does not develop freely his physical and mental energy but mortifies his body and ruins his mind. The worker therefore only feels himself outside his work, and in his work feels outside himself. He feels at home when he is not working, and when he is working he does not feel at home. His labour is therefore not voluntary, but coerced; it is forced labour. It is therefore not the satisfaction of a need; it is merely a means to satisfy needs external to it. Its alien character emerges clearly in the fact that as soon as no physical or other compulsion exists, labour is shunned like the plague. External labour, labour in which man alienates himself, is a labour of self-sacrifice, of mortification. Lastly, the external character of labour for the worker appears in the fact that it is not his own, but someone else's, that it does not belong to him, that in it he belongs, not to himself, but to another. Just as in religion the spontaneous activity of the human imagination, of the human brain and the human heart, operates on the individual independently of him—that is, operates as an alien, divine or diabolical activity—so is the worker's activity not his spontaneous activity. It belongs to another; it is the loss of his self. […] This relation [the relation of labour to the act of production] is the relation of the worker to his own activity as an alien activity not belonging to him; it is activity as suffering, strength as weakness, begetting as emasculating, the worker's own physical and mental energy, his personal life—for what is life but activity?—as an activity which is turned against him, independent of him and not belonging to him. Here we have self-estrangement, as previously we had the estrangement of the thing. Marx, Economic and Philosophic Manuscripts, Marx Engels Collected Works Volume 3 (International Publishers), p.274-275

In creating a world of objects by his practical activity, in his work upon inorganic nature, man proves himself a conscious species-being, i.e., as a being that treats the species as its own essential being, or that treats itself as a species-being. Admittedly animals also produce. They build themselves nests, dwellings, like the bees, beavers, ants, etc. But an animal only produces what it immediately needs for itself or its young. It produces one-sidedly, whilst man produces universally. It produces only under the dominion of immediate physical need, whilst man produces even when he is free from physical need and only truly produces in freedom therefrom. An animal produces only itself, whilst man reproduces the whole of nature. An animal's product belongs immediately to its physical body, whilst man freely confronts his product. An animal forms objects only in accordance with the standard and the need of the species to which it belongs, whilst man knows how to produce in accordance with the standard of every species, and knows how to apply everywhere the inherent standard to the object. Man therefore also forms objects in accordance with the laws of beauty. It is just in his work upon the objective world, therefore, that man really proves himself to be a species-being. This production is his active species-life. Through this production, nature appears as his work and his reality. The object of labour is, therefore, the objectification of man's species-life: for he duplicates himself not only, as in consciousness, intellectually, but also actively, in reality, and therefore he sees himself in a world that he has created. In tearing away from man the object of his production, therefore, estranged labour tears from him his species-life, his real objectivity as a member of the species, and transforms his advantage over animals into the disadvantage that his inorganic body, nature, is taken away from him. Marx, Economic and Philosophic Manuscripts, Marx Engels Collected Works Volume 3 (International Publishers), p.276-277

Marx adopts an instrumental disposition against nature in this manuscript.

An immediate consequence of the fact that man is estranged from the product of his labour, from his life activity, from his species-being is the estrangement of man from man. When man confronts himself, he confronts the other man. What applies to a man's relation to his work, to the product of his labour and to himself, also holds of a man's relation to the other man, and to the other man's labour and object of labour. In fact, the proposition that man's species-nature is estranged from him means that one man is estranged from the other, as each of them is from man's essential nature. The estrangement of man, and in fact every relationship in which man [stands] to himself, is realised and expressed only in the relationship in which a man stands to other men. Hence within the relationship of estranged labour each man views the other in accordance with the standard and the relationship in which he finds himself as a worker. Marx, Economic and Philosophic Manuscripts, Marx Engels Collected Works Volume 3 (International Publishers), p.277-278

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

Private property thus results by analysis from the concept of alienated labour, i.e., of alienated man, of estranged labour, of estranged life, of estranged man. True, it is as a result of the movement of private property that we have obtained the concept of alienated labour (of alienated life) in political economy. But analysis of this concept shows that though private property appears to be the reason, the cause of alienated labour, it is rather its consequence, just as the gods are originally not the cause but the effect of man's intellectual confusion. Later this relationship becomes reciprocal. Only at the culmination of the development of private property does this, its secret, appear again, namely, that on the one hand it is the product of alienated labour, and that on the other it is the means by which labour alienates itself, the realisation of this alienation. Marx, Economic and Philosophic Manuscripts, Marx Engels Collected Works Volume 3 (International Publishers), p.279-280