James Heartfield
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I got the impression that `power' as a direct adaptation from physics. That made labor power into a physical quantity that `drove' production and was magnified by machines to enhance `power'. With this general metaphor in mind, then man was the ultimate machine. Capital rented that power. Power becomes part of the whole system: horse power, steam power, labor power, etc. It helps add the dynamic element like a motive force.
CG