If I wish to make the claim, “Joe Blow killed Mrs. Schmidlap”, then I am making a POSITIVE assertion. Let’s say the defendant Mr. Blow makes a NEGATIVE statement, i.e., I did not kill Mrs. Schmidlap. In all formal rules of debate, it is the individual who makes the positive assertion that must present evidence to support their case. Someone who makes the negative statement need not. If you tell me, as George H. Smith once did, that you have a magic elf sitting on your head, it is not up to me to prove you don’t. It is up to you to prove you do. Of course - and here’s the catch - you can NEVER prove it’s there. I can never prove beyond a shadow of a doubt that you killed Mrs. Schmidlap. I might be able to state my case such that you do not have any REASONABLE doubt left. That is the highest standard, legally speaking.
The statement, “The defendant is presumed innocent until proven guilty beyond a reasonable doubt” is the whole statement. But “innocent until proven guilty” alone is a fallacy.
It is a common conception that “negatives can’t be proven”, i.e., prove that I was not having sex with your wife last night. But scientifically speaking (and in this case legally speaking too) disproof is the only kind of proof you get. It would be very literally impossible for a prosecutor to prove sensu stricto that someone committed a crime (a positive assertion), hence the presumption of innocence.
Why is this and what do I mean by it? Consider the proposition, as Karl Popper did, “All swans are white.” This is analagous to the statement “OJ Simpson killed his wife.” Both are positive statements based on evidence you may have gathered. All the swans you’ve ever seen are white. The glove had blood on it, DNA, etc. etc. But then one day, much to your surprise, you see a black swan. There goes your whole “all swans are white” theory. All it takes is one black swan, or one person other than Mrs. OJ with DNA exactly like hers (ha ha) to disprove your theory once and for all.
The same thing goes for the theory of gravity. Sure, every time I’ve dropped something it’s fallen down to the ground and I fully expect that this will keep happening. Millions of people, birds, cliffs, planets etc. have been dutifully following the rules and dropping, moving, etc. for billions of years. But I cannot make the positive statement “This pen will drop to the ground when I let it go”. You cannot say for sure what odd thing will occur a millisecond from now that will completely throw the theory of gravity for a loop. Am I pretty damned sure that won’t happen? Oh yes. Am I CERTAIN? Here I scratch my chin and must confess that I am not.
So “presumption” of innocence is just legal speak for, "you cannot conclusively demonstrate beyond a shadow of a doubt that Jow Blow is guilty. What you CAN demonstrate is that he might be to 12 presumably reasonable people. The presumption of innocence isn’t something we do today just to be nice to defendants. It’s based on a long history of forensic debate well known to our forefathers and given a serious backbone by some philosophers in the 1940’s and 1950’s. Those who make positive statements must back up their statements with evidence, and in our system, this means proving every element of a crime beyond a reasonable doubt.