Senior seeks to supply the unifying principle. "Abstinence" and labor have pain as a common element, and so are commensurable. Costs, reduced to labor and abstinence, become homogeneous again. Monism is restored. Cairnes completes the doctrine by adding risk to the real cost elements: a triune cost concept, sacrifice being the generic fact in the three manifestations. With John Stuart Mill, in general, we have an entrepreneur view-point. Money-expenses of production, entrepreneur outlay, plus wages of management, or including wages of management, are the factors with which Mill reckons. He is no longer concerned with psychological ultimates, or real costs. Cairnes criticised Mill sharply for this. No distinction is more fundamental he holds, than that between costs or sacrifice on the one hand, and rewards on the other. Labor, abstinence and risk are sacrifices; wages, interest, profits are rewards. None the less, in cost doctrine, as in supply and demand doctrine, it is Mill's view which has prevailed. Cost as conceived by Mill is a superficial, pecuniary notion. It tells little as to ultimate causation. But it is virtually only as a pecuniary doctrine, costs from the entrepreneur view-point, that the cost doctrine is met in modern theory. Why is this? Well, first, the real-cost doctrine simply does not square with the facts. The hardest labor does not produce the most valuable goods. Value in fact does not vary either with labor-pain or labor-time. In fact, whatever the explanation, it would seem to be truer that the relation is an inverse relation. Nor does the abstinence that pinches hardest produce the largest amount of capital. And while there is some correlation between risks and profits, the correlation is at best low and is not a correlation between psychological sacrifice and profits. Even "marginal abstinence" for a Rothschild or a Rockefeller causes no pain. It is absurd to seek to find a common element in the "abstinence" of a rich man and the pain of a poor and aged laborer. I pass over the supposed difficulty that abstinence is, in general, suffered by one set of minds, and labor-pain by a different set of minds, and hence, since men cannot compare their own emotions with the emotions of other men, there is no comparability. This subjectivistic psychology would, of course, make it equally impossible to fund labor-pains of different laborers, or to get any common denominator at all.[56] It is enough to point out that differences between rich and poor, between successful and unsuccessful, between efficient and inefficient, (apart from acquired differences which may be smoothed out by the "stored up labor-of-training" principle) make labor-pain, and marginal labor-pain, vary greatly from value, and make labor-pain, abstinence and risk quite incommensurable, and quite without fixed relation to value. Cairnes saw this in part, and developed his doctrine of non-competing groups to deal with it.