The Spirit of the Laws by Montesquieu is one of the inspirations for the United States Constitution. The legal philosophy text covers a few themes, one of which is the examination of morality.
Montesquieu believes that civil law should reflect what the people perceive as right and wrong, but he states that this is different from personal beliefs on morality. For example, someone who is impoverished might justify theft as a means to survive. For the thief, theft is a gray area.
However, in a society's civil law, theft is wrong regardless of circumstance. This objective approach by Montesquieu was influenced by Niccolò Machiavelli's political philosophy works. While others criticized the Machiavellian ethos as evil and draconian, Montesquieu believed that it is a rational and analytical way of viewing a society's constitution.
This basis allowed Montesquieu to avoid confirming a concrete solution to issues present in structuring a society's legal and political structure. Instead, he opines that each culture should configure their legal code and political structure according to their cultural beliefs and collective ideas on what is right and wrong.
The book meanders through different topics and sub-topics, and does not seem to have a solid structure in terms of thesis. However, this is due to Montesquieu's limited resources at the time. Despite the book's flaws, Montesquieu does a relatively good job of explaining the functions of the government and even discusses how it could be corrupted, and therefore details what should be avoided in maintaining strong governance.