8. It is not necessary, in view of our present purpose, to enter further into the nature of the law of reciprocal demand. The question of importance to us is the effect on the labor-cost theory of value of the doctrine of non-competing groups. Cairnes’s famous doctrine merely adds emphasis to a point already made by Senior, namely, that the wages of skilled labor are out of proportion to the amount of labor cost remunerated. Though Cairnes rejects the definition of cost as entrepreneur’s cost, his whole argument signifies that subjective cost, or pain cost, can control market value only by way of controlling the wages and interest elements which compose entrepreneur’s cost. As for the element of interest, Cairnes, like Senior, holding interest to be paid for abstinence, places it upon an independent basis of subjective cost, and makes it logically coördinate with wages; and Cairnes would treat the difficulty of interest, or the difficulty of “the different proportions of fixed and circulating capital,” not as Ricardo did, but precisely as Senior did.[191] As for the element of wages cost, the doctrine of non-competing groups signifies that the comparative wages cost of different commodities may fail to represent their comparative labor costs, or specifically, that they do so fail, when we compare the cost of commodities produced by different non-competing groups. Even if labor cost were the only kind of subjective cost that the entrepreneur has to remunerate, true labor cost would still fail to control exchange values, because the comparative wage expenses of entrepreneurs are not in proportion to the true amount of labor remunerated in each case. In the view of Cairnes, skill as such cannot be called an element in cost of production. But skill may be, and generally is, he continues, “an indication of that which is an element in cost—namely, the sacrifice whether in the form of labor or abstinence, undergone in acquiring the skill.... The point to be attended to is that the addition made to the cost of production [_i. e._ by the employment of skilled labor] is in proportion ... [only] to the sacrifice.”[192] If competition were effective between groups of laborers, it would result that wages would be forced to correspond with the disutility of labor in all employments, skilled or unskilled (the disutility of acquiring the skill being weighed by the competitors along with the daily disutility of the occupation). In the absence of effective competition—namely, under the actual conditions of non-competing groups—skilled labor generally receives a wage beyond that proportionate to the comparative disutility, past and present, incidental to it. Cairnes himself refused to regard his innovations as causing any substantial damage to the older theories, for of his whole doctrine of non-competing groups, he says: