The above formula will apply, with the appropriate changes of emphasis, to buyers and sellers in an organized market, as well as to the parties to a simple transaction of barter. Two main empirical characteristics of the economic situation are suggested in putting the statement in just these terms. In the first place, the primary problem in such a situation is that of "exchange valuation," the fixation of a "subjective" (or better, a "personal") price ratio between what the agent wishes to acquire and whatever it is that he offers in exchange. The agent thus is engaged in determining what shall be the relative importance for himself of _two_ commodities or exchangeable goods. And in the second place these goods get their values determined together and in relation to each other, never singly and with a view to _subsequent_ comparison. These values when they have been determined will be measured in terms of marginal utility in accordance with familiar principles, but the marginal utilities that are to express the attained and accepted ratio at which exchange eventually takes place are not known quantities at all in the inception of the process of comparison. If these dogmatic statements seem to issue in hopeless paradox or worse, then let us not fear to face the paradox and fix its lines with all possible distinctness. Can a man decide to offer so much of one commodity for so much of another unless he _first_ has settled what each is worth to him in some intelligible terms or other? And is not this latter in point of fact the real decision--at all events clearly more than half the battle? Does not the exchange ratio to which one can agree "leap to the eyes," in fact, as soon as the absolute values in the case have been once isolated and given numerical expression?