The ‘air of reality’ test governs whether a defence should be put to the jury. That test provides that the determination of putting a defence to the jury is based on “whether there is evidence on the record upon which a properly instructed jury acting reasonably could acquit”:
When a Trial Judge must put a Defence to the Jury: the Air of Reality Test
R. v. Cinous, 2002 SCC 29,
 2 S.C.R. 3, at para. 49.
The trial judge “must put to the jury all defences that arise on the facts”. It does not matter whether an accused advanced such a defence:
Cinous, at para. 51.
Nor does it matter if the defence is incompatible with defences advanced by the accused:
see R. v. Gauthier, 2013 SCC 32,  2 S.C.R. 403, at paras. 31-34.
At the same time, the trial judge must not put to the jury any defences that lack an air of reality: Cinous, at para. 51. As Cory J. explained, “[s]peculative defences that are unfounded should not be presented to the jury. To do so would be wrong, confusing, and unnecessarily lengthen jury trials”:
R. v. Osolin,  4 S.C.R. 595, at p. 683.
As the Supreme Court has made clear, “a mere assertion by the accused of the elements of a defence will not be sufficient to clear the air of reality hurdle”:
Cinous, at para. 98.
So, for example, where a victim has been stabbed multiple times by the accused, for there to be an air of reality to the defence of accident, the evidence would have to support an explanation as to how the accused could have stabbed the victim multiple times by accident. Otherwise, a properly instructed jury, acting reasonably, could not accept that the stabbings were accidental based on the factual foundation before them.
See for instance R. v. Cesar, 2016 ONCA 890.