What is the "air of reality" defence?

The common law has long recognized that a trial judge need not put to the jury defences for which there is no real factual basis or evidentiary foundation. Courts must filter out irrelevant or specious defences, since their primary effect would not be to advance the quest for truth in the trial, but rather to confuse finders of fact and divert their attention from factual determinations that are pertinent to the issue of innocence or guilt. Since this court’s judgment in Pappajohn v. The Queen (1980), 1980 CanLII 13 (SCC), 52 C.C.C. (2d) 481, 111 D.L.R. (3d) 1, [1980] 2 S.C.R. 120, the requirement that such a foundation exist for a defence before it is put to the jury has generally come to be known as the “air of reality” test.