What is the defence of mistaken belief in consent?

As I have pointed out on other occasions, this defence is rarely available, and this is not one of those rare cases when it is available. As Mclntvre J. said in Pappajohn v. The Queen (1980), 1980 CanLII 13 (SCC), 52 C.C.C. (2d) 481 at p. 514, 111 D.L.R. (3d) 1, [1980] 4 W.W.R. 387 at p. 424: To require the putting of the …defence of mistaken belief in consent, there must be …some evidence beyond the mere assertion of belief in consent by counsel for the appellant. This evidence must appear from or be supported by sources other than the appellant in order to give it any air of reality. He said further, at p. 515 C.C.C., p. 425 W.W.R.: Where the complainant says rape and the accused says consent, and where on the whole of the evidence, including that of the complainant, the accused, and the surrounding circumstances, there is a clear issue on this point, and where as here the accused makes no assertion of a belief in consent as opposed to an actual consent, it is unrealistic in the absence of some other circumstance or circumstances ... to consider the Judge bound to put the mistake of fact defence. In my opinion, the trial Judge was correct in refusing to put the defence on the evidence before him. These passages relate precisely to the evidence in this case.