Book 18 of Montesquieu's Spirit of the Laws consistently defies explication. Its 31 chapters seem to most commentators to constitute a heterogeneous melange, the parts bearing no or little relation to the whole—or, to express it still more precisely, there being no whole at all, there are consequently no parts. (Book 18 is the only book in which the number of chapters equals the number of books in the entire work. Its interpretative history parallels the history of the interpretation of The Spirit of the Laws. It is notable, moreover, that the last chapter added, chapter 28, Montesquieu prepared as he approached his deathbed, which act made the total of 31 chapters.) Thus, each chapter is treated as sui generis or, at most, as related only to a handful of others. The present account highlights the design, both within Book 18 and within Spirit of the Laws as a whole.

The Place of Book 18 in the Order of Spirit of the Laws

Book 18 opens with the Thucydidean declaration that the bounty or stinginess of the terrain determines, respectively, a people's tendency toward, on the one hand, dependence and subjection or, on the other hand, industry and liberty. After illustrating this principle with a few following chapters, Montesquieu declares in his own name (chapter 10) that a people's mode of subsistence determines the number of inhabitants in a land (or what is the same thing, the extent of territory required to support them). But such distinctions eventuate also in differences in the civil and political conditions of peoples (chapters 13–21). Finally, Montesquieu explains the derivation of what is ordinarily called the Salic law from the savage or barbarian circumstances and mode of existence of the Germanic peoples (chapters 22–31).

Most of the difficulty in Book 18 derives from the final 10 chapters, 22–31, which constitute a vigorous and ambitious re-statement of the primitive origins of French monarchy, under the guise of re-stating the origins of Salic laws. Thus, at least, the matter is generally conceived by commentators, who observe correctly that those chapters did not figure in the initial manuscript and, moreover, seem to every appearance to be related to the historical material in Book 28 and Books 30 and 31.