In the American justice system, convicting someone of a crime requires proof beyond reasonable doubt. As in the Casey Anthony trial, jurors must acquit if they merely think a defendant is guilty, but aren't sure. To convict, jurors must believe he or she is truly guilty; while they can never know it with absolute certainty, their doubt cannot be of "reasonable" proportions.

Unsurprisingly, the issue of how much doubt is "reasonable" and thus, legally, must lead to an acquittal is extremely murky. Yale legal historian James Q. Whitman put it this way in a recent article for the History News Network: "Exactly when are legal 'doubts' about the guilt of the accused 'reasonable'? Jurors are sometimes understandably baffled. Even some of the most sophisticated members of the legal profession find the question too difficult to answer."

In American courtrooms, juries are instructed to apply the reasonable doubt standard when determining their verdicts. They are often not told what this means, however. When they are, even the definition itself is nebulous; in many legal documents it goes something along the lines of "a real doubt, based upon reason and common sense after careful consideration of all the evidence." Some law dictionaries add the colloquialism, "you better be damned sure." [Read: Why Is the Constitution So Difficult to Interpret? ]

Historic reversal

In today's world, the reasonable doubt standard tends to favor the accused by placing a strict burden of proof on the prosecution. But it came into use in medieval England for just the opposite reason. "[The] 'beyond a reasonable doubt' standard was not originally designed to make it more difficult for jurors to convict. It was originally designed to make conviction easier, by assuring jurors that their souls were safe if they voted to condemn the accused," Whitman wrote in his book "The Origins of Reasonable Doubt" (Yale University Press 2007).

Jurors believed that wrongly condemning an innocent person was a mortal sin. If, after hearing a trial, they felt any uncertainty whatsoever about the defendant's guilt or innocence, they tended not to convict in fear of eternal repercussions. According to Whitman, the standard Christian law held that "in every case of doubt, where one's salvation is in peril, one must always take the safer way. . . . A judge who is in doubt must refuse to judge."

The reasonable doubt doctrine was thus introduced to urge jurors to be more willing to convict; it says, effectively, "you may have a tiny bit of doubt, but come, let's be reasonable here: the defendant is guilty."

Follow Natalie Wolchover on Twitter @Nattyover. Follow Life's Little Mysteries on Twitter @llmysteries, then join us on Facebook.