http://classiques.uqac.ca/classiques/montesquieu/de_esprit_des_lois/partie_1/de_esprit_des_lois_1.html

OK let's consider Montesquieu the Magnificent a bit.

Let's look near the beginning of the work, to start.

Part 1, Book VI, Chapter 16 -- On making the punishment fit the crime
Chapter 17 -- On putting criminals to the torture

Chapter 16 makes the far from startling observation that it is necessary for more severe crimes to result in more severe punishments in order for criminal law to be effective. Several examples are cited from contemporary European history, and also from as far afield as contemporary China and Russia, to prove this point, all rigorously footnoted and srupulously referenced.

While this observation is certainly true, it is curious that Montesquieu completely fails to note a very well known and very old illustration of this basic point that would certainly be familiar to every single one of his readers -- the Lex Talonis of the Mosaic Law, from the Old Testament of the Bible. It is derived from the oldest legal code, the Code of Hammurabai, from ancient Mesopotamia, and is the earliest and most basic concept in all law.

There are two possible explanations for this omission on Montesquieu's part:
1. He hadn't the imagination to make this very obvious parallel and universal generalization to the Biblical Law.
2. He wanted to claim credit for having made some kind of original legal discovery, based on original research.

I suspect both explanations are to some extent correct. Montesquieu has both the virtues and vices of the academic/scholar. He is thorough, systematic and rigorous. He is also unimaginative, self-important lacking in basic common sense.

Let's go on to Chapter 17, I'll translate the whole thing:

On putting criminals to the Torture or "Question"

"Because people are by nature evil, the law is obliged to consider them to be better than they are. Thus, the testimony of two witnesses is sufficient to result in conviction for any and all crimes. The law believes them, as if they spoke from the mouth of truth itself. It is also judged in law that all children conceived in marriage are legitimate: the law has confidence in the mother as if she were chastity itself. But there is no such necessity to put criminals to the "Question". We can see today a very well regulated state reject it without undue inconvenience. It is, therefore, not by nature necessary."

"So many clever people, so many brilliant men of genius have written against this practice, that I do not presume to write further of it. I would say that it might be appropriate in despotic societies, where the mechanisms of government are based primarily on inspiring fear, for example amongst the slaves in ancient Greece and Rome... but I hear the voice of nature itself crying against me."

This passage is so bad that it reads more like an argument for torture rather than -- as is its apparent intention -- against it! Montesquieu is, in this particular case, unable to transcend the ignorance of his own time regarding the most basic principles of social dynamics that we understand today. The problem with state sponsored torture is that its social behavioral effects are far more brutalizing and socially disruptive than are the vast majority of crimes. Montesquieu does not understand this point -- obvious to the contemporary mind -- because the sciences of sociology and psychology did not exist in the eighteenth century.

Montesquieu is, thus, limited both by his academic nature and his eighteenth century ignorance of basic principles of society. This is why he is not much referred to today, unlike universal geniuses such as Euclid and Newton.