As soon as man, instead of working with an implement on the subject of his labour, becomes merely the motive power of an implement-machine, it is a mere accident that motive power takes the disguise of human muscle
 Karl Marx, Das Kapital (1867). copy citation

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Author Karl Marx
Source Das Kapital
Topic power work
Date 1867
Language English
Reference
Note Translated by Samuel Moore and Edward Aveling
Weblink https://www.marxists.org/archive/marx/works/download/pdf/Capital-Volume-...

Context

“The steam-engine itself, such as it was at its invention, during the manufacturing period at the close of the 17th century, and such as it continued to be down to 1780,9 did not give rise to any industrial revolution. It was, on the contrary, the invention of machines that made a revolution in the form of steam-engines necessary. As soon as man, instead of working with an implement on the subject of his labour, becomes merely the motive power of an implement-machine, it is a mere accident that motive power takes the disguise of human muscle; and it may equally well take the form of wind, water or steam. Of course, this does not prevent such a change of form from producing great technical alterations in the mechanism that was originally constructed to be driven by man alone.” source