What is the test for a mistake of fact in the context of criminal negligence?

McIntyre J. then went on to consider the question of a mistake of fact as it relates to criminal negligence at pp. 141-2: The application of an objective test under s. 202 of the Criminal Code, however, may not be made in a vacuum. Events occur within the framework of other events and actions and when deciding on the nature of the questioned conduct, surrounding circumstances must be considered. The decision must be made on a consideration of the facts existing at the time and in relation to the accused’s perception of those facts. Since the test is objective, the accused’s perception of the facts is not to be considered for the purpose of assessing malice or intention on the accused’s part but only to form a basis for a conclusion as to whether or not the accused’s conduct, in view of his perception of the facts, was reasonable. This is particularly true where, as here, the accused have raised the defence of mistake of fact. If an accused under s. 202 has an honest and reasonably held belief in the existence of certain facts, it may be a relevant consideration in assessing the reasonableness of his conduct. For example, a welder, who is engaged to work in a confined space, believing on the assurance of the owner of the premises that no combustible or explosive material is stored nearby, should be entitled to have his perception, as to the presence or absence of dangerous materials, before the jury on a charge of manslaughter when his welding torch causes an explosion and a consequent death. … It was held in this court in Pappajohn v. The Queen, 1980 CanLII 13 (SCC), [1980] 2 S.C.R. 120, that an honest, though mistaken, belief in the existence of circumstances which, if present, would make the questioned conduct non-culpable would entitle an accused to an acquittal. It was also held in Pappajohn that the honest belief need not be reasonable, because its effect would be to deny the existence of the requisite mens rea. The situation would be different, however, where the offence charged rests upon the concept of negligence, as opposed to that of the guilty mind or blameworthy mental state. In such case, an unreasonable though honest belief on the part of the accused would be negligently held. The holding of such a belief could not afford a defence when the culpability is based on negligent conduct. I would therefore conclude that the trial judge made no error in charging the jury to the effect that any mistaken belief which could afford a defence in a charge of criminal negligence would have to be reasonable. (emphasis added)