I graduated from Harvard Law School in 1956, practiced law for a couple of years and then moved into investment banking. That is my modest way of saying: beware of my expertise. Yet, I did learn quite a bit that non-lawyers might struggle with when they are confronted, as we all are today, with numerous legal processes in the daily news.

When Special Counsel are rummaging around in a vast body of history such as the Trump case, they are trying to tease out a lot of facts that tie things together—or do not—to find incriminating information about some of the actors. They then sometimes indict these actors and secure guilty pleas or convictions all aimed at building towards their ultimate task, target, or subject of inquiry.

As they are doing that, they have to be constantly conscious of the “best evidence” rule.

No, that does not necessarily mean the “best” (or most convincing) evidence in support of their case; instead, it means the best reliable source or type of evidence needed by a trier of the facts. For example, hearsay or testimonial evidence is rarely, if ever, sufficient to bolster an important and damaging claim against a defendant. Documents, photos or contemporaneous data (e.g., GPS tracking) clearly showing a story or facts are far better than the recollections of any eyewitness.

The best evidence rule places a heavy burden on a prosecutor to outdo the defense. And the defense has to do its best to overcome the type of evidence produced by the prosecution.

Why is this important in the Trump case?

Start with the Stormy Daniels hush money; paid just days before the 2016 election by Trump’s lawyer, using what he claimed was his own money (while waffling on possibly being subsequently reimbursed). On its face that statement is almost beyond belief—lawyers just do not do that and in most jurisdictions they are explicitly barred from using personal funds on a client’s behalf.

All the Special Counsel had on the facts was the record of the financial transaction, proving that the payment had been made (perhaps violating some banking rules). Trump has denied knowing anything about it. Cohen—the lawyer—said only he knew about it. Stormy disagreed with both of them. That left the Special Counsel with an impasse and effectively no evidence to refute Trump’s and Cohen’s assertions.

That is why the Special Counsel, with the agreement of the Deputy Attorney General, referred the case to the US attorney’s office in New York where, with the approval of a Federal District Court judge, based on evidence in their possession in support of their request, they sought and received permission to raid Cohen’s offices in pursuit of contradictory, presumably deliberately hidden evidence to rebut Trump’s and Cohen’s statements.

It was not only that they were seeking better evidence; they were seeking any type of evidence that could meet the best evidence rule, which they are obliged to try to find, if they were to do their required job thoroughly.

One of the factors now being used to attack the Special Counsel in that case is whether attorney-client privilege protects everything in Cohen’s possession. The simple answer is that the lawyer-client privilege only applies to evidence that came to him while he was actually acting as a lawyer for Trump. Since Trump claims not to have known about it, and Cohen claims Trump didn’t know about it, any evidence the FBI might have found in the raid, by definition, isn’t related to Cohen’s legal work for Trump.

Also complicating matters is that Cohen was formerly a Trump employee, as well as at times, his lawyer. When he was what, is, of course, a bit of a tangle which will have to be sorted out regarding any other evidence that might emerge from the raid. But that question was surely well considered in advance. And, the nature of whatever was found will help a judge decide what was what. Anything covered by an attorney-client privilege will have to be given back and never used – that’s why it is standard FBI procedure to have the information seized in a raid and reviewed by people isolated from the investigation itself. That too was surely well considered in advance of the raids.

So the American public is in for a bit of legal education, if they can find it within the obfuscation of talking heads, complicit members of Congress and the fiery, and largely fact-free, rants of our president.

We are proud that America, for almost 250 years, has been governed by the rule of law, and lucky that internal threats to it have been rare. We cannot, though, have it both ways. If we want to continue to be a nation of laws, rather than of men (mostly), we must let the legal processes run its course and try hard to understand all the ins and outs.