>From my pov, the "manual" labor can be as if not more intellectually
challenging than many white collar jobs.
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Absolutely. For one thing, you have to struggle with reality...rather than just manipulating symbols.
But part of the problem is the gradual de-skilling of manual labor. So far as I can see, this does not necessarily improve efficiency, but it does render the worker more helpless and more ignorant.
Joanna