What is the defence of due diligence?

[64] In commenting on the defence of due diligence in his appellate decision in Regina v. Nitrochem Inc., [1993] O.J. No. 3336 (Ont. Prov. Ct.), Anderson J. stated as follows: An analysis of the defence reveals two possible arguments. First it is open to the accused to establish that he reasonably believed in a mistaken set of facts which if true would have rendered the act or omission harmless. The second branch of the argument is that the accused did everything reasonably within his power to avoid the event. In practical terms however, the two aspects of the defence ultimately converge since to establish the reasonableness of the mistaken belief an inquiry is necessary to determine whether the accused did everything reasonably within his power to ascertain the true state of affairs.