In his last book, “The Wisdom of the World,” Remi Brague, a historian of philosophy who teaches at the Sorbonne, explored a part of our intellectual heritage so remote that it now seems almost incredible. That was the idea, dead since Copernicus, that the very structure of the cosmos is the mirror of God — that by studying the paths of the stars and planets, we gain insight into the divine nature, and into our own. Today, when religious believers challenge the reign of science, it is biology that is most likely to be their target. Darwin, not Copernicus or Galileo, is the man they hold responsible for the estrangement of man’s moral nature from his physical nature. In fact, as Mr. Brague brilliantly showed, it was astronomy, not biology, that for millennia seemed the discipline by which man could reach God. Once we have conceded, as even the most religious now do, that our planet lies in an obscure corner of a planless universe, the fact that we are descended from the apes hardly seems worth getting worked up about.

Now, in “The Law of God” (University of Chicago Press, 264 pages, $35), Mr. Brague undertakes another journey through the buried continent of the ancient and medieval mind. But his topic this time — the idea of divine law, as it was understood from the ancient Greeks through the Muslim, Christian, and Jewish middle ages — does not seem nearly so remote. Humanity has long conceded that the structure of the inanimate world is the province of science. But most of us continue to believe that the moral law has other, deeper sources. The idea that God gives us laws — whether in the form of the Ten Commandments, the Jewish Halakhah, the Muslim Sharia, or simply the voice of conscience — is one that the world is understandably, perhaps rightfully, reluctant to give up.

That is why “The Law of God” strikes the reader with more intimate force than “The Wisdom of the World.” Mr. Brague’s earlier book was archaeology, the digging up of something dead and buried; his new one is genealogy, tracing the descent of ideas that are still living. His method, and some of his conclusions, are similar to those of Leo Strauss, about whom Mr. Brague has written in the past. He performs close readings of major philosophical texts, trying to draw out the sometimes unspoken understandings that inform them. Using this method, Mr. Brague compares the way Athens understood divine law with the way Jerusalem understood it. He goes on to examine the evolution of concepts of law in Judaism, Christianity, and Islam, through the high middle ages.

Finally, in an all too brief summary chapter, he shows how the idea of divine law has decomposed in the modern era. Today, we easily think of laws in terms of the laws of nature, which are really descriptions and equations. The law of gravity or the law of relativity are not laws in the proper sense, which the universe could choose to obey or not. They are simply ways of describing what always happens. On the other hand, we can understand human laws as self-imposed ethical or political obligations, which have a purely conventional, positive status — they are laws because we say they are laws. What we find hard to imagine are commandments — laws that possess authority because they come from God.

Our current understanding of law, Mr. Brague shows, is equally distant from the understanding of the ancient Greeks and that of the ancient Israelites. The Greeks often spoke of “divine law,” but seldom about a “law of God,” in the sense of a code handed down from a personal deity. For them, Mr. Brague writes, “the adjective ‘divine’ indicates no trace of an origin in a god designated by a substantive, even less by a proper noun.” The law was divine, not because Jove or Apollo wrote it, but because it belonged to the nature of things. It expressed the essence of man and of nature, and the way they fit together. For Cicero, paraphrasing the teaching of the Stoics, law is “the highest reason, implanted in Nature.”

“Greek divine law,” Brague explains, “is divine because it expresses the profound structures of a permanent natural order. Jewish law,” on the other hand, “is divine because it emanates from a god who is master of history.” In Exodus and again in Deuteronomy, the Law is represented as being written by God’s own hand: On Sinai, God gives Moses “two tables of testimony, tables of stone, written with the finger of God.” By making law the vehicle of God’s covenant with man, the Jewish Bible effectively broke with the practice of Egypt and Babylon, which made the king himself God’s representative. Readers of the Book of Samuel will remember that the anointing of Israel’s first king, Saul, is depicted in surprisingly negative terms. Israel got a king, it seems, only because the people were too weakwilled to let the law itself rule them.

It was not until Israel lost its political sovereignty, however, that Judaism, with its distinctive conception of the law, was born. The heart of “The Law of God” is found in Mr. Brague’s comparison of the three monotheistic faiths with regard to their conceptions of divine law. But because Judaism was the religion of a people without a state, Mr. Brague is interested in it less for its own sake than as a precursor to Christianity and Islam, where the conflicts between religion and politics were more acute.

Mr. Brague writes ostensibly as a historian of ideas, not a defender of any religious tradition. But the further one reads in “The Law of God,” the clearer it becomes that the purpose of the book is to demonstrate the superiority of the Christian conception of law to the Muslim conception. In neutral, scholarly language, Mr. Brague recapitulates the exact same binarisms that have informed European views of Islam since the Enlightenment. You can see the trend of his argument simply by looking at the subheadings in the text. In Brague’s discussion of Christianity, there are sections on “Consience,” “Autonomy,” and “The Law of Love”; in his discussion of Islam, the equivalents are “Restoration,” “Confirmation,” and “Falsification”

In other words, Christianity is a religion of grace, revolving around the free soul; Islam is a religion of obedience, concerned with imposing its authority. The former is creative and open-ended, the latter regimented and closed-off. The same kind of familiar opposition informs Mr. Brague’s treatment of Judaism. His chapter on medieval Judaism is called “The Law as an End”; the corresponding chapter on Christianity is “The End of the Law.”

Mr. Brague makes a strong case for such characterizations, through his close readings of the New Testament and the Koran, and of medieval treatises and commentaries. But it is hard not to be suspicious of an interpretation that hews so completely to ancient stereotypes and that confirms the European Christian or post-Christian reader in his accustomed sense of superiority. One would like to know how far Mr. Brague’s antinomies reflect the actual historical experience of Christian, Muslim, and Jewish believers. I wonder, for instance, whether most Christians at most times would have felt, as Mr. Brague suggests, that “for Christianity the relationship to the law is not one of ‘being under’ it; it is not one of subordination.”

Part of the problem is that Mr. Brague focuses exclusively on formal philosophical statements about the law, thus dispensing with the necessity of immersing the reader in actual legal traditions. He pays little attention, that is, to the content or form of the Talmud or Sharia, or the canon law of the medieval church. Instead, he dwells on the theoretical statements of master thinkers like Maimonides, Thomas Aquinas, and Ghazali.

This gives him great interpretive freedom, including the freedom to be tendentious, in a way that his own sources might not be able to endorse or even recognize. As with Strauss, it is sometimes hard to tell where analysis leaves off and interpretation begins. But as with Strauss, also, Mr. Brague’s sense of intellectual adventure is what makes his work genuinely exciting to read. “The Law of God” offers a challenge that anyone concerned with today’s religious struggles ought to take up.

akirsch@nysun.com