Reasonable Doubt
All criminal trials turn on the issue of "reasonable doubt." If reasonable doubt exists, a jury cannot convict the defendant. Prior to a jury's attempt to reach a verdict, the court instructs it that the defendant is presumed innocent and that the presumption remains intact unless guilt, beyond a reasonable doubt, is found.
There is no official definition of "reasonable doubt" that is used in criminal cases, and many courts do not define it for the jury at all. Reasonable doubt has been described as "the real possibility" that the defendant is not guilty, or indecision so great that the jury does not have a certainty of guilt. It is not necessary that all doubt be extinguished, or that a scientific certainty of the defendant's guilt exists in order for a jury to convict.
The prosecution has the burden to prove guilt beyond a reasonable doubt in all criminal cases. At no time during trial does the burden of proof shift to the defendant to prove innocence. The defendant is never required to prove innocence, nor present any facts that might raise reasonable doubt in the minds of the jury.