Part 3 discusses climate and terrain to show how geophysical and atmospheric conditions affect the law, the mores, and the general spirit of the state. Montesquieu is especially concerned with slavery and the effect of climate on human character. In Book 14, he extends the “general idea” of Part 3: “If it is true that the character of the spirit and the passions of the heart are extremely different in the various climates, laws should be relative to the differences in these passions and to the differences in these characters” (231). The author intends, then, to determine how laws should relate to climate, which ostensibly has a powerful effect on a people’s character.

Montesquieu begins with a sweeping generalization: “The peoples in hot countries are timid like old men; those in cold countries are courageous like young men” (232). He bases this conjecture on the belief that colder climates desensitize people to pleasure, while the hotter climates of the southern regions sensitize people to love and make people lazy: “As you move toward the countries of the south,” he writes, “you will believe you have moved away from morality itself” (234).