" The issue here turned on the work content of a day's labor
power, which Taylor defines in the phrase "a fair day's work."
To this term he gave a crude physiological interpretation: all
the work a worker can do without injury to his health, at a
pace that can be sustained throughout a working lifetime. (In
practice, he tended to define this level of activity at an extreme
limit, choosing a pace that only a few could maintain, and
then only under strain.) Why a "fair day's work" should be
defined as a physiological maximum is never made clear. In
attempting to give concrete meaning to the abstraction
"fairness," it would make just as much if not more sense to
express a fair day's work as the amount of labor necessary to
add to the product a value equal to the worker's pay; under
such conditions, of course, profit would be impossible. The
phrase "a fair day's work" must therefore be regarded as
inherently meaningless, and filled with such content as the
adversaries in the purchase-sale relationship try to give it. "

― Harry Braverman , Labor and Monopoly Capital: The Degradation of Work in the Twentieth Century