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.