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.
Karl MarxWe know only a single science, the science of history. History can be contemplated from two sides, it can be divided into the history of nature and the history of mankind. However, the two sides are not to be divided off; as long as men exist the history of nature and the history of men are mutually conditioned.
Karl MarxNature is man's inorganic body -- that is to say, nature insofar as it is not the human body. Man lives from nature -- i.e., nature is his body -- and he must maintain a continuing dialogue with it is he is not to die. To say that man's physical and mental life is linked to nature simply means that nature is linked to itself, for man is a part of nature.
Karl Marx