[WS:] That was not my point at all. Mental labor can be very challenging as well. I do a lot of data analysis and a sloppy code can be very costly indeed.
My point was that first, certain types of mental work is less forgiving than manual work, in the sense that slacking off and bullshitting does not have a clearly visible consequences as, say, a botched plumbing system. Second, there is no such thing as purely "manual" work (except maybe jerking off ;)) - a lot of blue collar jobs require not only a lot of knowledge but also the ability to apply this knowledge, be creative in problem solving, etc.
Wojtek