In 1767 Forbonnais published the Principes et observations oeconomiques with the declared intent of challenging the scientific validity of the Tableau économique, despite the fact that in 1759 ‘he had been one of its keenest readers, just as Quesnay had been of his Recherches et considérations sur les finances de France depuis l'année 1595 jusqu’à l'année 1721’. Indeed, the relationship between Quesnay, the court physician who had given himself to the study of the economy, and Forbonnais, the inspecteur général des monnaies de France (from 1756) and economic advisor to the contrôleur général Étienne de Silhouette, was strong, as the letters that the former sent to the latter during 1758 attest. In one dated 14 September, Quesnay had expressed his appreciation for Forbonnais's ideas ‘on circulation’, saying these were ‘the only way to conceive of it in reality and truth’. Quesnay praised him for having avoided the trap of tracing the origins of the wealth of a nation by taking into account only the circulation of money. However, in the same letter he invited Forbonnais to follow a new path to analyse that circulation: in carrying out his research and ‘assessment’ Forbonnais needed to bear in mind
that the richesses usuelles are no more than a flow of foodstuffs always destroyed by consumption and replenished by reproduction; that what is income or profit for some subjects and expenditure or loss for others is not an income for the state, but only a redistribution of income; and that there is no revenue for the state except from agricultural products that can be traded, and from the net profit of commerce with foreigners.
Quesnay also stated that it was necessary to distinguish between
the net profit of foreign trade as a sales instrument whose effect is to support the market value of goods and manufactured products, the manufactured products being considered nothing more than a way to allow agricultural products to flow.
In essence, despite continuing to maintain that foreign trade could lead to a net profit, in this letter to Forbonnais, Quesnay had anticipated the key arguments of his Tableau in which, significantly, the possibility that foreign trade could produce a net profit disappeared.