In Maybin v R, 2012 SCC 24 (18 May 2012) Karakatsanis J for the Court discusses the criteria for determining that D's acts have caused the actus reus, here the death of V, to an extent that criminal responsibility could be attributed to D. Essentially this is a moral question.
The legal requirement is that D's acts were a significant contributing cause of the death. There may be several causes of the death, but if D's contribution was more than de minimis, causation will be established for the purpose of deciding whether D is criminally responsible.
Legal responsibility requires that D's acts had sufficient connection to that harm to warrant attribution of criminal responsibility [16]. There might be an insufficient connection if there was an intervening act so that D's acts were not a significant contributing cause of the harm. Such intervening acts might not sever the causal link between D's acts and the harm if they should have been foreseen by D at the time he acted [26], and this does not mean that D should have foreseen the exact details of the intervening act, but some sort of appreciation of what might happen, akin perhaps to the mens rea requirements of the offence, may be enough to make D responsible [36], [38].
The independent acts might not sever the causal connection between D's acts and the harm if, although those independent acts were unforeseeable, the relative weights of the causes of the harm [46] were such as to leave a more than minimal causal link attributable to D.
In any event, these are just analytical aids, not legal tests, for determining causal responsibility. In different factual matrices they might point in different ways. The fundamental question is always whether D's acts were a significant contributing cause of the harm.
The moral question whether D should be held responsible when there are multiple causes of a harm will often attract different answers, as is illustrated in the differences of opinion in the Court of Appeal here. There had been a fight in a bar and D had knocked out V, who was then struck by a third person, a bouncer. V died and D was charged with manslaughter. You be the judge (after reading the full facts in the case, of course).
