In the 18th century, the production of books was still an essentially manual process. By means of a hand press, large sheets of paper were printed on both sides with multiple pages before being folded. They were folded once to produce four pages for the folio size, twice to produce eight pages for the quarto or four times to produce the 16-page octavo. These folded sheets, known as "gatherings," were then sewn together along their inner edges before being attached to the binding. To ensure that the bookbinders would stitch the gatherings together in the correct sequence, each was marked with a different letter of the alphabet on what, after folding, would become that gathering's first page.

Thus, in an octavo volume like Jefferson's Qur'an, there is a small printed letter in the bottom right-hand corner of every 16th page. It was Jefferson's habit to take advantage of these preexisting marks to discreetly inscribe each of his books. On each book's 10th gathering, in front of the printer's mark J he wrote a letter T, and on the 20th gathering, to the printed T he added a J, thereby in each case producing his initials. This subtle yet unmistakable signature appears clearly on the two leather-bound volumes in the Library of Congress.

Jefferson's system of cataloging his library sheds light on the place the Qur'an held in his thinking. Jefferson's 44-category classification scheme was much informed by the work of Francis Bacon (1561–1626), whose professional trajectory from lawyer to statesman to philosopher roughly prefigures Jefferson's own career. According to Bacon, the human mind comprises three faculties: memory, reason and imagination. This trinity is reflected in Jefferson's library, which he organized into history, philosophy and fine arts. Each of these contained subcategories: philosophy, for instance, was divided into moral and mathematical; continuing along the former branch leads to the subdivision of ethics and jurisprudence, which itself was further segmented into the categories of religious, municipal and "oeconomical."

Jefferson's system for organizing his library has often been described as a "blueprint of his own mind." Jefferson kept his Qur'an in the section on religion, located between a book on the myths and gods of antiquity and a copy of the Old Testament. It is illuminating to note that Jefferson did not class religious works with books on history or ethics—as might perhaps be expected—but that he regarded their proper place to be within jurisprudence.

The story of Jefferson's purchase of the Qur'an helps to explain this classification. Sifting through the records of the Virginia Gazette, through which Jefferson ordered many of his books, the scholar Frank Dewey discovered that Jefferson bought this copy of the Qur'an around 1765, when he was still a student of law at the College of William & Mary in Virginia. This quickly refutes the notion that Jefferson's interest in Islam came in response to the Barbary threat to shipping. Instead, it situates his interest in the Qur'an in the context of his legal studies—a conclusion that is consistent with his shelving of it in the section on jurisprudence.

Jefferson's legal interest in the Qur'an was not without precedent. There is of course the entire Islamic juridical tradition of religious law (Shari'ah) based on Qur'anic exegesis, but Jefferson had an example at hand that was closer to his own tradition: The standard work on comparative law during his time was Of the Law of Nature and Nations, written by the German scholar Samuel von Pufendorf and first published in 1672. As Dewey shows, Jefferson studied Pufendorf's treatise intensively and, in his own legal writings, cited it more frequently than any other text. Pufendorf's book contains numerous references to Islam and to the Qur'an. Although many of these were disparaging—typical for European works of the period—on other occasions Pufendorf cited Qur'anic legal precedents approvingly, including the Qur'an's emphasis on promoting moral behavior, its proscription of games of chance and its admonition to make peace between warring countries. As Kevin Hayes, another eminent Jefferson scholar, writes: "Wanting to broaden his legal studies as much as possible, Jefferson found the Qur'an well worth his attention."

In his reading of the Qur'an as a law book, Jefferson was aided by a relatively new English translation that was not only technically superior to earlier attempts, but also produced with a sensitivity that was not unlike Jefferson's own emerging attitudes. Entitled The Koran; commonly called the Alcoran of Mohammed, it was prepared by the Englishman George Sale and published in 1734 in London. A second edition was printed in 1764, and it was this edition that Jefferson bought. Like Jefferson, Sale was a lawyer, although his heart lay in oriental scholarship. In the preface to his translation, he lamented that the work "was carried on at leisure time only, and amidst the necessary avocations of a troublesome profession." This preface also informed the reader of Sale's motives: "If the religious and civil Institutions of foreign nations are worth our knowledge, those of Mohammed, the lawgiver of the Arabians, and founder of an empire which in less than a century spread itself over a greater part of the world than the Romans were ever masters of, must needs be so." Like Pufendorf, Sale stressed Muhammad's role as a "lawgiver" and the Qur'an as an example of a distinct legal tradition.

This is not to say that Sale's translation is free of the kind of prejudices against Muslims that characterize most European works on Islam of this period. However, Sale did not stoop to the kinds of affronts that tend to fill the pages of earlier such attempts at translation. To the contrary, Sale felt himself obliged to treat "with common decency, and even to approve such particulars as seemed to me to deserve approbation." In keeping with this commitment, Sale described the Prophet of Islam as "richly furnished with personal endowments, beautiful in person, of a subtle wit, agreeable behaviour, shewing liberality to the poor, courtesy to every one, fortitude against his enemies, and, above all, a high reverence for the name of God." This portrayal is markedly different from those of earlier translators, whose primary motive was to assert the superiority of Christianity.

In addition to the relative liberality of Sale's approach, he also surpassed earlier writers in the quality of his translation. Previous English versions of the Qur'an were not based on the original Arabic, but rather on Latin or French versions, a process that layered fresh mistakes upon the errors of their sources. Sale, by contrast, worked from the Arabic text. It was not true, as Voltaire claimed in his famous Dictionnaire philosophique of 1764, that le savant Sale had acquired his Arabic skills by having lived for 25 years among Arabs; rather, Sale had learnt the language through his involvement in preparing an Arabic translation of the New Testament to be used by Syrian Christians, a project that was underwritten by the Society for the Promotion of Christian Knowledge in London. Studying alongside Arab scholars who had come to London to assist in this work, he acquired within a few years such good command of the language that he was able to serve as a proofreader of the Arabic text.

It is thus not so surprising that Sale turned from translating the holy text of Christians into Arabic to rendering the holy text of Muslims into his native English. Noting the absence of a reliable English translation, he aimed to provide a "more genuine idea of the original." Lest his readers be unduly daunted, he justified his choice of fidelity to the original by stating that "we must not expect to read a version of so extraordinary a book with the same ease and pleasure as a modern composition." Indeed, even though Sale's English may appear overwrought today, there is no denying that he strove to convey some of the beauty and poetry of the original Arabic.

Sale's aspiration to provide an accurate rendition of the Qur'an was matched by his desire also to provide his readers with a more honest introduction to Islam. This "Preliminary Discourse," as he entitled it, runs to more than 200 pages in the edition Jefferson purchased. Fairly presented and conscientiously documented, it contains a section on Islamic civil law that repeatedly points out parallels to Jewish legal precepts in regard to marriage, divorce, inheritance, lawful retaliation and the rules of warfare. In this substantial discussion, Sale displays the same quality of dispassionate interest in comparative law that later moved Jefferson.

But did reading the Qur'an influence Thomas Jefferson? That question is difficult to answer, because the few scattered references he made to it in his writings do not reveal his views. Though it may have sparked in him a desire to learn the Arabic language (during the 1770's Jefferson purchased a number of Arabic grammars), it is far more significant that it may have reinforced his commitment to religious freedom. Two examples support this idea.

In 1777, the year after he drafted the Declaration of Independence, Jefferson was tasked with excising colonial legacies from Virginia's legal code. As part of this undertaking, he drafted a bill for the establishment of religious freedom, which was enacted in 1786. In his autobiography, Jefferson recounted his strong desire that the bill not only should extend to Christians of all denominations but should also include "within the mantle of its protection, the Jew and the Gentile, the Christian and Mahometan [Muslim], the Hindoo, and infidel of every denomination."

This all-encompassing attitude to religious pluralism was by no means universally shared by Jefferson's contemporaries. As the historian Robert Allison documents, many American writers and statesmen in the late 18th century made reference to Islam for less salutary aims. Armed with tendentious translations and often grossly distorted accounts, they portrayed Islam as embodying the very dangers of tyranny and despotism that the young republic had just overcome. Allison argues that many American politicians who used "the Muslim world as a reference point for their own society were not concerned with historical truth or with an accurate description of Islam, but rather with this description's political convenience."

These attitudes again came into conflict with Jefferson's vision in 1788, when the states voted to ratify the United States Constitution. One of the matters at issue was the provision—now Article vi, Section 3—that "no religious Test shall ever be required as a Qualification to any Office or public Trust under the United States." Some Anti-Federalists singled out and opposed this ban on religious discrimination by painting a hypothetical scenario in which a Muslim could become president. On the other side of the argument, despite their frequent opposition to Jefferson on other matters, the Federalists praised and drew on Jefferson's vision of religious tolerance in supporting uncircumscribed rights both to faith and to elected office for all citizens. As the historian Denise Spellberg shows in her examination of this dispute among delegates in North Carolina, in the course of these constitutional debates "Muslims became symbolically embroiled in the definition of what it meant to be American citizens."

It is intriguing to think that Jefferson's study of the Qur'an may have inoculated him—to a degree that today we can only surmise— ainst such popular prejudices about Islam, and it may have informed his conviction that Muslims, no less and no more than any other religious group, were entitled to all the legal rights his new nation could offer. And although Jefferson was an early and vocal proponent of going to war against the Barbary states over their attacks on us shipping, he never framed his arguments for doing so in religious terms, sticking firmly to a position of political principle. Far from reading the Qur'an to better understand the mindset of his adversaries, it is likely that his earlier knowledge of it confirmed his analysis that the roots of the Barbary conflict were economic, not religious.

Sale's Koran remained the best available English version of the Qur'an for another 150 years. Today, along with the original copy of Jefferson's Qur'an, the Library of Congress holds nearly one million printed items relating to Islam—a vast collection of knowledge for every new generation of lawmakers and citizens, with its roots in the law student's leather-bound volumes.