By Noah Feldman / Bloomberg Opinion

We still don’t know a lot of important facts about President Biden’s retention of classified documents at his Penn-Biden center office and Delaware home. For that matter, there’s a lot we still don’t know about former president Donald Trump’s retention of classified documents at Mar-a-Lago. What is certain is the legal concept that prosecutors will have to use to determine whether to prosecute either president: intent.

The relevant provision of the Espionage Act is fairly clear. It’s a crime if you both “willfully” retain classified documents and also “fail” to deliver them “on demand” to the government official “entitled” to receive them.

The adverb “willfully” is standard legal language, a close cousin to “knowingly.” To act willfully, in the legal sense, is to act intentionally, consciously and voluntarily. As the Department of Justice has put it, relying on judicial interpretation of statutes, “An act is done ‘willfully’ if done voluntarily and intentionally and with the specific intent to do something the law forbids.”

In practice, this requirement of willfulness could very well mean that Trump committed a crime with respect to the classified material he retained, while Biden did not. Consider that Trump, through representatives, allegedly refused for months to return more than 30 boxes of documents sought by the National Archives; a refusal that eventually prompted the FBI to show up at Mar-a-Lago and seize the material. That refusal sounds very much like it matches the language of the Espionage Act, which requires both unauthorized retention of classified documents and the refusal to hand them over.

In contrast, according to what we know so far, Biden retained — perhaps by accident — what’s been described as a “small number” of classified documents after leaving the vice presidency. When the documents were found, his lawyers appear to have promptly handed them over without even having to be asked. If true, that would not violate the relevant Espionage Act provision.

To a trained lawyer, or a devout watcher of “Law & Order,” this distinction, which could be determinative for prosecution, is intuitive. Almost all crimes come with a definitional requirement of willful or voluntary behavior.

To non-lawyers, this legal focus on intent might seem strange. How do you prove anyone willfully does anything? And why bother?

The deep answer to these questions begins with the moral theory of crime. We generally think the law should criminalize conduct that is morally wrongful, not just socially undesirable. And if you aren’t voluntarily and consciously doing something, then we tend to think you aren’t as morally culpable for doing it as you would be if you knew what you were doing.

From this moral foundation springs our mechanism of proof. Investigators look at the circumstances of an action to determine if it was done consciously and voluntarily.

Those circumstances are often much clearer than you might think. Did Trump know he was retaining boxes of documents and refusing to hand them over? We may not be able to read Trump’s mind. But we can still answer that legal and moral question by considering what Trump did when the National Archives asked for its documents.

And we can make a solid determination of Biden’s willfulness, or lack of it, by examining what he did when his lawyers found the classified documents in his office and home.

At this point, you may notice that the two special prosecutors in the Trump and Biden situations are being called on to make moral judgments before they make legal ones. In effect, they’re being asked to figure out whether the two men did anything morally wrong. The answer to that will shape the legal answer of whether to prosecute.

That’s how it’s supposed to work.

Intent is fundamental to the criminal law because it’s fundamental to our sense of right and wrong. If our legal system works, Trump and Biden’s conduct will each get the appropriate level of fact-specific moral and legal scrutiny.

Noah Feldman is a Bloomberg Opinion columnist. A professor of law at Harvard University, he is author, most recently, of “The Broken Constitution: Lincoln, Slavery and the Refounding of America.”