ABSTRACT
The existence of the presumption is beyond dispute, but in one respect the traditional formulation of the presumption calls for re-examination. This respect concerns the position of a defendant who acted under a mistaken view of the facts. In this regard, the presumption is expressed traditionally to the effect that an honest mistake by a defendant does not avail him unless the mistake was made on reasonable grounds. Thus, in R v Tolson (1889) 23 QBD 168, 181, Cave J observed:
At common law an honest and reasonable belief in the existence of circumstances, which, if true, would make the act for which a prisoner is indicted an innocent act has always been held to be a good defence. This doctrine is embodied in the somewhat uncouth maxim ‘actus non facit reum, nisi mens sit rea’. Honest and reasonable mistake stands on the same footing as absence of the reasoning faculty, as in infancy, or perversion of that faculty, as in lunacy ... So far as I am aware it has never been suggested that these exceptions do not equally apply in the case of statutory offences unless they are excluded expressly or by necessary implication.
