As the trial of Elizabeth Holmes opens, it’s important to remember one thing: She’s innocent.

I’m not saying the founder of Theranos didn’t commit fraud in marketing the company’s quick and easy blood test. Maybe she did. Certainly I’ve expressed skepticism about aspects of her defense strategy. But whatever opinions you or I might form outside the courtroom, inside she’s presumed innocent unless proven guilty beyond a reasonable doubt. That’s why the trial is necessary.

The “presumption of innocence” is a curious turn of phrase. Outside the courtroom, it’s long provided useful fodder for those in favor of bail reform or against civil-asset forfeiture: People are not presumed innocent, the argument goes, if they’re punished before trial. Fair enough.

On the other hand, the presumption hardly dissuades the public from reaching conclusions, often strongly held ones, about the guilt or innocence of defendants in prominent trials. Opinion polls about their guilt or innocence are routine, and have been for more than a quarter century.

So perhaps this is a good moment for a quick primer on why the public’s view of a trial is often quite different from the jury’s perspective.

On the first day of evidence class, I tell my students that until trial begins, nobody knows anything. What I mean by this is that, unlike news reports, a criminal trial is governed by rules of evidence. Details that “everybody knows” might also be details that the jury never hears or is instructed to ignore. The latest juicy leak might be deemed inadmissible.

In the case of Holmes’s trial, for example, it’s all very well for the news media to report that patients are expected testify to having received false assurances about Theranos’s technology, or that Holmes claims to have been psychologically coerced into doing whatever she did. But the outcome of the trial will depend on what unfolds in the courtroom — what precise testimony the judge admits, whether the defendant takes the stand, and so on. And this search for truth is guided by (and confounded by) the presumption of innocence.

The proposition that the defendant is presumed innocent until proven guilty is usually attributed to Sir William Garrow, arguing at the Old Bailey in London in 1791, although versions of the phrase appear earlier. And the general concept that it is the accuser, not the defendant, who must prove the case has roots not only in Roman law but also in the great Western religions.

What does the presumption mean in practice? In Scott Turow’s 1987 novel “Presumed Innocent,” the presiding judge in a murder trial asks a prospective juror whether or not the defendant committed “the crime that he is charged with.” The man shrugs and replies — as I suspect most of us would — “I wouldn’t know.” The judge excludes him from the jury, then turns to the remaining potential jurors and explains: “Ladies and gentlemen, let me tell you again what you are to presume. ... Presume he is innocent. I want you to look over and say to yourself, ‘There sits an innocent man.’”

Not every judge would state the matter quite so baldly, but the scene makes the point. When a criminal case becomes notorious, most people form opinions on the guilt of innocence of the accused. Often these opinions are strongly held, even well-reasoned. But as one legal scholar has put it: “What is rational for a casual observer is not permissible for a juror.”

In the courtroom, jurors are asked to set their views aside. They begin the trial with the presumption of innocence, which the U.S. Supreme Court described in 1895 as “evidence in favor of the accused.”

This means that from the first day of her trial, before a single witness has been heard, there’s evidence of Holmes’s innocence and none of her guilt. Inside the courtroom, on Day One, the likelihood that she is guilty should be close to zero.

Ideally, as the trial proceeds, the jurors will adjust their estimates. Each piece of evidence should increase or decrease the confidence of the jury in the defendant’s innocence. This process asks a certain cognitive humility. Were we in Eden still, perhaps all of us would behave that way in everyday life; in the world where we actually live, none of us quite do.

But jurors must. In this sense, the perfect trial tests the null hypothesis that there exists no connection between the defendant and the crime. The hypothesis can be falsified only when the prosecution presents sufficient admissible evidence to enable the jury to reject the hypothesis beyond reasonable doubt.

Those of us outside the courtroom need not subscribe to this hypothesis. But the jury has a different task, which explains why trials so often end in ways the public does not expect. In the courtroom, what matters is what the admissible evidence shows. Until the prosecution is forced to make a case, the null hypothesis is true.

Reams of social science cast doubt on whether this ideal is attainable in practice. But if the goal is to avoid punishing the innocent, we should strive to bring it as near to reality as possible. Thus the jurors in Holmes’s trial should begin by looking at the defendant and saying to themselves: “There sits an innocent woman.”

This column does not necessarily reflect the opinion of the editorial board or Bloomberg LP and its owners.

Stephen L. Carter is a Bloomberg Opinion columnist. He is a professor of law at Yale University and was a clerk to U.S. Supreme Court Justice Thurgood Marshall. His novels include “The Emperor of Ocean Park,” and his latest nonfiction book is “Invisible: The Forgotten Story of the Black Woman Lawyer Who Took Down America’s Most Powerful Mobster.”

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