RUTHFULLY YOURS

That rule was the attorney-client privilege. It compromised the public’s otherwise unrelenting claim to every man’s evidence so as to encourage full and frank communication between lawyers and clients. The trade-off was that legal disputes were a search for the truth, and we would wind up with more truth if we didn’t search the parties’ lawyers to get it. Otherwise no client would disclose any unhelpful truth to a lawyer, and the process would collapse.

The privilege has a limit, the crime-fraud exception. The privilege is not recognized if the client is using the lawyer to commit a crime. For the most part this exception is read narrowly, to cover only the motive of the client. If the lawyer is acting on his own—for the client’s benefit but without the client’s authorization—evidence of the lawyer’s conduct would not be privileged, although evidence of the client’s statements would remain so.

That’s tidy in the abstract, and if a lawyer is served with a subpoena for information. But things get messy when, as occurred here, the party seeking the information is the government, and the means of getting it is a search warrant.

As to any particular client, the lawyer’s records will include myriad information, some of it client disclosures, some concerning conduct undertaken with the approval of the client—all of that privileged. There may also be records of the lawyer’s conduct and statements in pursuance of a criminal scheme, on his own or with the client—none of it privileged.

The government agents executing the warrant must isolate records that are relevant and unprivileged. They have no obvious way to do that other than to look at all records they seize, which means necessarily that they will look at records that should not be subject to a government search.

Agents typically do that by sending in a “filter team,” whose members have no contact with those involved in the investigation. That ensures that information that should not be examined—whether privileged or not—does not seep into, or “taint,” the investigation.

That’s the theory. Its success depends on the skill and honor of the filter team, and even in the best case it doesn’t prevent some government personnel from examining private records that are none of the government’s business.

The process risks damaging the significant public interest that led to the creation of the privilege in the first place. Is it worth the reward? One would think that would depend at least in part on the seriousness of the crimes under investigation.

In this case, the government has redacted any reference to particular crimes, although we learned earlier that perhaps Donald Trump’s lawyer, Michael Cohen, spent $130,000 on the eve of the 2016 election to buy the silence of Stephanie Clifford, a k a Stormy Daniels, about a sexual encounter she claimed she had with Mr. Trump several years ago. The payment itself would violate no law, but if its sole motive was to improve Mr. Trump’s chances in the election, it could constitute an in-kind campaign contribution that was not disclosed, in violation of campaign laws. The amount, more than $25,000, would put the violation in the felony category.

Have such violations been vigorously prosecuted in the past? In 2012 President Obama received campaign donations exceeding $2 million from sources that were not disclosed; he received another $1.3 million that exceeded contribution limits. The matter was settled after the election by the Obama Justice Department for a fine of $375,000 and no felony prosecution.

Another possible crime said to justify the search warrant is bank fraud. Here, one theory is that Mr. Cohen might have defrauded his bank by falsifying the purpose of the home-equity loan that was reportedly the source of the $130,000. But if Mr. Cohen’s equity is sufficient to provide collateral for the loan, why would its purpose matter to the bank? And if it doesn’t matter, there’s no fraud.

We can’t be certain that this is all that is under investigation. But if it is, the potential gains from an intrusive and unusual search warrant look meager compared with the interest put at risk. How confident are we in the self-control of those who set these events in train—who after all were supposed to be at work protecting the integrity of our elections?

Oddly, an episode involving a poison no doubt familiar to all three madmen mentioned in the first paragraph gives pause.

After anthrax spores killed five people, infected 17 others, and showed up in envelopes mailed to U.S. senators and media organizations in 2001, the current special counsel, then director of the Federal Bureau of Investigation, spent years chasing and destroying the reputation of a microbiologist named Steven Hatfill, zealous in the belief that Mr. Hatfill was the guilty party. Another zealot, James Comey, then deputy attorney general, said he was “absolutely certain” no mistake had been made.

After Mr. Hatfill was exonerated—he received more than $5.5 million in damages from the government—Mr. Mueller then decided that another microbiologist, Bruce Ivins, was the culprit. When Ivins committed suicide, Mr. Mueller pronounced the case closed. A subsequent investigation by the National Academy of Sciences suggests Ivins too was innocent.

Mr. Mueller is not a bad man, nor is Mr. Comey. It’s just that both show particular confidence when making mistakes, which makes one grateful for safeguards like the attorney-client privilege.

Mr. Mukasey served as U.S. attorney general (2007-09) and a U.S. district judge (1988-2006).