Much the most important rule in criminal law is that expressed in the Latin maxim “semper praesumitur pro negante,” This is the well-known presumption of innocence. It is the same as the frequently invoked principle that in order to secure a conviction the evidence for the prosecution must be such as not merely to show a probability of the defendant’s guilt, but to leave no reasonable doubt of it (R. v. Beale, 8 Cr. App. R. 95). In other words, the onus lies on the prosecution to establish its case beyond any reasonable doubt. By way of exception to this rule, it is well recognised now that there are cases in which the onus lies on the defendant to prove an affirmative, particularly in the case of crimes of omission where the facts would be peculiarly within the defendant’s knowledge. In one case it was apparently thought that the rule as to the onus of proof in criminal cases was wide enough to embrace even these cases. In Over v. Harwood [19 ....