Geoffroi Jacques Flach (February 16, 1846 – December 4, 1919) was a French jurist and historian born at Strasbourg, Alsace, of a family known at least as early as the 16th century, when Sigismond Flach was the first professor of law at University of Strasbourg.
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Flach studied classics and law at Strasbourg, and in 1869 took his degree of doctor of law.
The Franco-Prussian War engaged Flach's activities in other directions, and he spent two years (described in his Strasbourg après le bombardment, 1873) at work on the rebuilding of the library and the museum, which had been destroyed by Prussian shells.
When the time came for him to choose between Germany and France, he settled definitely in Paris, where he completed his scientific training at the École des Chartes and the École des Hautes Études.
A celebrated lawsuit in Alsace, pleaded by his friend and compatriot Ignace Chauffour, aroused his interest by reviving the question of the origin of the feudal laws, and gradually led him to study the formation of those laws and the early growth of the feudal system.
His great work, Les Origines de l'ancienne France, was produced slowly.
It explains how the Gallo-Roman villa gave place to the village, with its fortified castle, the residence of the lord; how new towns were formed by the side of old, some of which disappeared; how the townspeople united in corporations; and how the communal bond proved to be a powerful instrument of cohesion.
At the same time it traces the birth of feudalism from the germs of the Gallo-Roman personal comitalus; and shows how the bond that united the different parties was the contract of the fief; and how, after a slow growth of three centuries, feudalism was definitely organized in the 12th century.
In 1904 appeared the third volume, La Renaissance de l'État, in which the author describes the efforts of the Direct Capetian kings to reconstruct the power of the Frankish kings over the whole of Gaul; and goes on to show how the clergy, the heirs of the imperial tradition, encouraged this ambition; how the great lords of the kingdom (the "princes", as Flach calls them), whether as allies or foes, pursued the same end; and how, before the close of the 12th century, the Capetian kings were in possession of the organs and the means of action which were to render them so powerful and bring about the early downfall of feudalism.
In these three volumes, which appeared at long intervals, the author's theories are not always in complete harmony, nor are they always presented in a very luminous or coherent manner, but they are marked by originality and vigour.
Flach gave them a solid basis by the wide range of his researches, utilizing charters and cartularies (published and unpublished), chronicles, lives of saints, and even those dangerous guides, the chansons de geste.
He owed little to the historians of feudalism, who knew what feudalism was, but not how it came about.