What constitutes the alienation of labour?

Firstly, the fact that labour is external to the worker – i.e., does not belong to his essential being; that he, therefore, does not confirm himself in his work, but denies himself, feels miserable and not happy, does not develop free mental and physical energy, but mortifies his flesh and ruins his mind. Hence, the worker feels himself only when he is not working; when he is working, he does not feel himself. He is at home when he is not working, and not at home when he is working. His labour is, therefore, not voluntary but forced, it is forced labour. It is, therefore, not the satisfaction of a need but a mere means to satisfy needs outside itself. […] Finally, the external character of labour for the worker is demonstrated by the fact that it belongs not to him but to another, and that in it he belongs not to himself but to another. Just as in religion the spontaneous activity of the human imagination, the human brain, and the human heart, detaches itself from the individual and reappears as the alien activity of a god or of a devil, so the activity of the worker is not his own spontaneous activity.

It belongs to another, it is a loss of his self.

The result is that man (the worker) feels that he is acting freely only in his animal functions – eating, drinking, and procreating, or at most in his dwelling and adornment – while in his human functions, he feels to be nothing more than animal. What is animal becomes human and what is human becomes animal.