Rueff’s mémoire defines exchange as a measurable natural mechanism and uses it to interpret the postwar monetary world. Its key term is “disparité”: the gap between a currency’s internal purchasing power, inferred from wholesale prices, and its purchasing power abroad, inferred from the exchange rate. The argument begins from a qualified purchasing-power-parity rule. Among countries not undergoing continuous fiduciary inflation, exchange tends toward a level that gives money comparable command over goods at home and abroad:
la monnaie du pays (1) s'échange contre celle du pays (2) à un cours qui lui donnera, à l'intérieur du pays (2), un pouvoir d'achat sensiblement égal à celui qu'elle possède à l'intérieur du pays (1).
English translation: The currency of country (1) is exchanged against that of country (2) at a rate that gives it, within country (2), a purchasing power appreciably equal to that which it possesses within country (1).
That qualification excludes cases such as Germany and Russia, where ongoing paper expansion prevents exchange from expressing a stable relation between prices and external value. For France, Britain, the United States, Italy, Spain, Belgium, and Switzerland, deviations from parity are not anomalies but the functional margin through which adjustment occurs. Disparity makes imports dearer or cheaper, changes the profitability of exports, and so acts on the balance of immediately payable foreign debts and claims. The exchange rate does not create the balance; it translates pressure into incentives that tend to alter trade flows:
Les variations de la disparité de la monnaie du pays (1) dans chacun des autres pays ont pour effet de maintenir l'équilibre de la balance des comptes du pays (1), ou de le rétablir lorsqu'il a été accidentellement troublé.
English translation: Variations in the disparity of country (1)'s currency in each of the other countries have the effect of maintaining the equilibrium of country (1)'s balance of payments, or of restoring it whenever it has been accidentally disturbed.
Rueff’s proof is deliberately statistical. He compares the franc’s purchasing power in France and abroad during 1920–22, tests the method against 1912–13 under gold circulation, and extends the comparison to sterling, the dollar, and several bilateral relations. The point is not mechanical simultaneity: lags are expected because orders, payments, and recorded trade do not coincide. What matters is the correspondence of turning points between disparity and commercial balance. The method itself is part of the claim: if economic units are badly chosen, aggregate data appear chaotic; if purchasing power is measured correctly, a regular law becomes visible.
The book’s title receives its theoretical justification in the second part. Rueff assimilates exchange adjustment to the law of displaced equilibrium associated with Van’t Hoff, Le Châtelier, and Lenz: when an equilibrium is disturbed, a system reacts in a direction that tends to counteract the disturbance. Prices coordinate dispersed actions without collective intention, and exchange rates extend the same function across borders by altering, for one country’s buyers, the relative level of another country’s goods.
La vie économique, dans son ensemble, et pour un groupe important d'individus, paraît bien ainsi régie par la loi la plus générale de la nature.
English translation: Economic life as a whole, and for an important group of individuals, thus appears indeed to be governed by the most general law of nature.
“Natural” therefore means neither ethically superior nor politically untouchable. It means that exchange is a stabilizing form whose effects can be studied like other equilibrating phenomena. Rueff’s liberalism follows from this diagnosis: intervention may be justified by security, justice, or diplomacy, but it cannot abolish the mechanism without paying the price of frustrated adjustment. France’s postwar export recovery illustrates the point, since a modest positive disparity of the franc abroad could induce commercial movements large enough to offset an apparent deficit.
The final part applies the theory to unemployment, debt, and reparations. British unemployment is connected to a negative external disparity of sterling, which weakened export profitability. International debt service is ultimately a transfer of goods, not merely of money, because monetary payments set in motion exchange-rate changes and trade flows that supply the real counterpart. Reparations are treated in the same way. Germany can be made to pay only if it is also allowed to export; fiscal measures can collect internal purchasing power, and exchange depreciation can help generate the external surplus, but inflation and import barriers against German goods block the mechanism. Rueff’s maxim is blunt:
Les deux prétentions d'obliger l'Allemagne à payer et de l'empêcher d'exporter sont contradictoires, donc absurdes.
English translation: The two claims of compelling Germany to pay and of preventing her from exporting are contradictory, and therefore absurd.
The conclusion is less a defense of laissez-faire as dogma than a demand that policy respect causal structure. Exchange rates, disparities, and trade balances form an impersonal coordinating system. A later note refines the terminology from “balance des comptes” toward “balance des paiements” and concedes variable lags between trade entries and exchange purchases. These corrections do not alter the architecture: for Rueff, economics becomes scientific when theory, statistical observation, and properly defined units converge, and international exchange is the exemplary case of that convergence.
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