The 'plateau of Tatary' that exists only in their heads
Editorial contextSome 18th-century writers had argued that all Old World civilisation began on a high plateau in the middle of Tatary. Klaproth, discussing the Uighurs, says their little kingdom never counted for much and gives no support to that idea, and he says bluntly that the plateau exists only in the brains of those who believe in it.
The favourite hypothesis of those who would have all the civilisation of the Old World come from the plateau of Tatary, which exists only in their brains, therefore receives no support from the Uighurs, whose little kingdom never rose to any political importance
Working translation
Original wording
L'hypothèse favorite de ceux qui veulent que toute la civilisation de l'ancien continent soit venue du plateau de la Tatarie, qui n'existe que dans leur cerveau, ne reçoit donc aucun appui de la part des Ouigour, dont le petit royaume ne s'est jamais élevé à une certaine importance politique
About Uighurs (a Turkic people of the eastern Tian Shan)
Keep in mind
Klaproth is arguing against earlier scholars (notably Bailly); this is his judgement, not an observation.