An extraordinary fuss about eavesdropping started in the spring of 1844, when Giuseppe Mazzini, an Italian exile in London, became convinced that the British government was opening his mail. Mazzini, a revolutionary who’d been thrown in jail in Genoa, imprisoned in Savona, sentenced to death in absentia, and arrested in Paris, was plotting the unification of the kingdoms of Italy and the founding of an Italian republic. He suspected that, in London, he’d been the victim of what he called “post-office espionage”: he believed that the Home Secretary, Sir James Graham, had ordered his mail to be opened, at the request of the Austrian Ambassador, who, like many people, feared what Mazzini hoped—that an insurrection in Italy would spark a series of revolutions across Europe. Mazzini knew how to find out: he put poppy seeds, strands of hair, and grains of sand into envelopes, sealed the envelopes with wax, and sent them, by post, to himself. When the letters arrived—still sealed—they contained no poppy seeds, no hair, and no grains of sand. Mazzini then had his friend Thomas Duncombe, a Member of Parliament, submit a petition to the House of Commons. Duncombe wanted to know if Graham really had ordered the opening of Mazzini’s mail. Was the British government in the business of prying into people’s private correspondence? Graham said the answer to that question was a secret.

Questions raised this month about surveillance conducted by the National Security Agency have been met, so far, with much the same response that Duncombe got from Graham in 1844: the program is classified. (This, a secret secret, is known as a double secret.) Luckily, old secrets aren’t secret; old secrets are history. The Mazzini affair, as the historian David Vincent argued in “The Culture of Secrecy,” led to “the first modern attack on official secrecy.” It stirred a public uproar, and eventually the House of Commons appointed a Committee of Secrecy “to inquire into the State of the Law in respect of the Detaining and Opening of Letters at the General Post-office, and into the Mode under which the Authority given for such Detaining and Opening has been exercised.” In August of 1844, the committee issued a hundred-and-sixteen-page report on the goings on at the post office. Fascinating to historians, it must have bored Parliament silly. It includes a history of the delivery of the mail, back to the sixteenth century. (The committee members had “showed so much antiquarian research,” Lord John Russell remarked, that he was surprised they hadn’t gone all the way back to “the case of Hamlet, Prince of Denmark, who opened the letters which had been committed to his charge, and got Rosencrantz and Guildenstern put to death instead of himself.”)

The report revealed that Mazzini’s mail had indeed been opened and that there existed something called the Secret Department of the Post Office. Warrants had been issued for reading the mail of the king’s subjects for centuries. Before Mazzini and the poppy seeds, the practice was scarcely questioned. It was not, however, widespread. “The general average of Warrants issued during the present century, does not much exceed 8 a-year,” the investigation revealed. “This number would comprehend, on an average, the Letters of about 16 persons annually.” The Committee of Secrecy was relieved to report that rumors that the Secret Department of the Post Office had, at times, sent “entire mail-bags” to the Home Office were false: “None but separate Letters or Packets are ever sent.”

The entire episode was closely watched in the United States, where the New-York Tribune condemned the opening of Mazzini’s mail as “a barbarian breach of honor and decency.” After the Committee of Secrecy issued its report, Mazzini published an essay called “Letter-Opening at the Post-Office.” Two months after the Mazzini affair began, the Secret Department of the Post Office was abolished. What replaced it, in the long run, was even sneakier: better-kept secrets.

The opening of Mazzini’s mail, like the revelations that the N.S.A. has been monitoring telephone, e-mail, and Internet use, illustrates the intricacy of the relationship between secrecy and privacy. Secrecy is what is known, but not to everyone. Privacy is what allows us to keep what we know to ourselves. Mazzini considered his correspondence private; the British government kept its reading of his mail secret. The A.C.L.U., which last week filed a suit against the Obama Administration, has called the N.S.A.’s surveillance program a “gross infringement” of the “right to privacy.” The Obama Administration has defended both the program and the fact that its existence has been kept secret.

As a matter of historical analysis, the relationship between secrecy and privacy can be stated in an axiom: the defense of privacy follows, and never precedes, the emergence of new technologies for the exposure of secrets. In other words, the case for privacy always comes too late. The horse is out of the barn. The post office has opened your mail. Your photograph is on Facebook. Google already knows that, notwithstanding your demographic, you hate kale.

The particular technology matters little; the axiom holds. It’s only a feature, though, of a centuries-long historical transformation: the secularization of mystery. A mystery, in Christian theology, is what God knows and man cannot, and must instead believe. Immortality, in this sense, is a mystery. So is the beginning of life, which is a good illustration of how much that was once mysterious became secret and then became private. Anciently invoked as one of God’s mysteries, the beginning of life was studied, by anatomists, as the “secret of generation.” Finally, citizens, using the language of a constitutional “right to privacy,” defended it against intrusion. Theologically, the beginning of life, the ensoulment of new flesh, remains a mystery. Empirically, uncovering the secret of generation required tools—microscopes, lenses, cameras—that made the creation of life both visible and knowable. Only after it was no longer a mystery, and no longer a secret, only after it was no longer invisible, did it become private. By then, it was too late: contraception was already in the hands of the state.

Secret government programs that pry into people’s private affairs are bound up with ideas about secrecy and privacy that arose during the process by which the mysterious became secular. The mysteries of the Church are beyond the knowledge of any man and, therefore, outside the scope of the state. During the Reformation, Protestants rejected many mysteries as superstitions, and what was mysterious then began to move from priests to princes. By the seventeenth century, the phrase “mysteries of state” meant both state secrets and monarchical power and right—not what God knows, and we do not know and must accept, but what the king knows, and we do not. In 1616, in a speech to the Star Chamber, James I talked about his “Prerogative or mystery of State,” proclaiming, “That which concernes the mysterie of the Kings power, is not lawfull to be disputed.” But monarchical notions about the royal prerogative were challenged by the very existence of books like “The Cabinet-Council, Containing the Cheif Arts of Empire and Mysteries of State, Discabineted,” published in 1658. It was an age of political reformation, rich with arguments that knowledge that was once the privilege of the king ought to be revealed, taken out of the king’s cabinet. In the early modern world, a mystery came to mean any kind of secret that could be revealed to an ordinary man.