They say: Political economy is not affected by a priori arguments; it pronounces only upon facts. Now, facts and experience teach us that there is no measure of value and can be none, and prove that, though the conception of such an idea was necessary in the nature of things, its realization is wholly chimerical. Supply and demand is the sole law of exchange. I will not repeat that experience proves precisely the contrary; that everything, in the economic progress of society, denotes a tendency toward the constitution and establishment of value; that that is the culminating point of political economy--which by this constitution becomes transformed--and the supreme indication of order in society: this general outline, reiterated without proof, would become tiresome. I confine myself for the moment within the limits of the discussion, and say that SUPPLY and DEMAND, held up as the sole regulators of value, are nothing more than two ceremonial forms serving to bring useful value and exchangeable value face to face, and to provoke their reconciliation. They are the two electric poles, whose connection must produce the economical phenomenon of affinity called EXCHANGE. Like the poles of a battery, supply and demand are diametrically opposed to each other, and tend continually to mutual annihilation; it is by their antagonism that the price of things is either increased, or reduced to nothing: we wish to know, then, if it is not possible, on every occasion, so to balance or harmonize these two forces that the price of things always may be the expression of their true value, the expression of justice. To say after that that supply and demand is the law of exchange is to say that supply and demand is the law of supply and demand; it is not an explanation of the general practice, but a declaration of its absurdity; and I deny that the general practice is absurd. I have just quoted Ricardo as having given, in a special instance, a positive rule for the comparison of values: the economists do better still. Every year they gather from tables of statistics the average prices of the various grains. Now, what is the meaning of an average? Every one can see that in a single operation, taken at random from a million, there is no means of knowing which prevailed, supply--that is, useful value--or exchangeable value,--that is, demand. But as every increase in the price of merchandise is followed sooner or later by a proportional reduction; as, in other words, in society the profits of speculation are equal to the losses,--we may regard with good reason the average of prices during a complete period as indicative of the real and legitimate value of products. This average, it is true, is ascertained too late: but who knows that we could not discover it in advance? Is there an economist who dares to deny it?