It “generally happens that the bulk of the value of each commodity follows one law, ... while a small remaining element is governed” by the other law.[187] This unsatisfactory idea is at times better expressed in another way, namely, the law of costs governs a good’s value in exchange with commodities produced by the same order of laborers, but not in exchange with commodities produced by different orders. “The true conception of the law of cost is thus, not of a law governing universally the values of any class of commodities, but that of one governing the values of certain commodities in certain exchanges.”[188] 7. We come now to the second coördinate principle of value, existing in addition to the principle of cost of production, and held to divide with it the control of actual values. This is the so-called law of reciprocal demand. It is simply Mill’s law of reciprocal demand as developed by him to explain international values, applied by Cairnes to the “strictly parallel case” of inter-group values. International and inter-group values, or the relative prices of the products of different nations or of different groups, “do not vary at random irrespective of rule or measure, but exhibit precisely the same tendency to gravitate towards a central point as is manifested in those exchanges which are governed by cost of production.”[189] These words surely attribute a very exact control of values to the “principle of reciprocal demand.” But after perusing a very interesting piece of text (pp. 99-105), in which Cairnes applies Mill’s doctrine of international value, point by point, to the problem of non-competing groups, we come upon a very important difference between this principle and that of cost in respect to control of values.