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 his mind. The worker therefore only feels himself outside his work, and in his work feels outside himself.

Estranged Labour, p. 30. - Paris Manuscripts (1844)