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Most Americans know that a criminal defendant is presumed innocent unless/until he is proven guilty.  Many people also understand that proof of a defendant’s guilt must be “beyond a reasonable doubt.”

While these are simple concepts to grasp, things become less clear when we try to explain what’s meant by proof “beyond a reasonable doubt.”

In fact, many able jurists and scholars have advised against such efforts.

As Australian judge Sir Edmund Barton once remarked

One embarks on a dangerous sea if he attempts to define with precision a term which is in ordinary use with reference to this subject-matter, and which is usually stated to a jury without embellishment as a well understood expression.

These sentiments were shared by the American legal scholar Professor John Wigmore who wrote

The term ‘reasonable doubt’ is almost incapable of any definition which will add much to what the words themselves imply.

Wigmore on Evidence, § 2497

But such warnings have been ignored by many American courts, including those in Massachusetts.  Here in the Bay State, judges presiding over criminal trials must read (verbatim) a lengthy description of “proof beyond a reasonable doubt” to jurors.  The description, in part, reads

What is proof beyond a reasonable doubt? The term is often used and probably pretty well understood, though it is not easily defined. Proof beyond a reasonable doubt does not mean proof beyond all possible doubt, for everything in the lives of human beings is open to some possible or imaginary doubt. A charge is proved beyond a reasonable doubt if, after you have compared and considered all of the evidence, you have in your minds an abiding conviction, to a moral certainty, that the charge is true. When we refer to moral certainty, we mean the highest degree of certainty possible in matters relating to human affairs — based solely on the evidence that has been put before you in this case.

Jury Instruction 2.180

The language used in the Massachusetts jury instruction is nearly identical to an 1850 ruling made by the Supreme Judicial Court in Commonwealth v. Webster, 59 Mass. (5 Cush.) 295, 320 (1850).

The U.S. Supreme Court in In re Winship explicitly held

That the Due Process Clause protects the accused against conviction except upon proof beyond a reasonable doubt of every fact necessary to constitute the crime with which he is charges.

In re Winship, 397 US 358.