The Supreme Court of Canada has ruled that accused people cannot claim drunkenness as a defence to crimes such as arson, assault and other offences typically associated with drinking, unless they are drunk to the point of automatism.

The ruling came in a case involving a jealous ex-boyfriend who got drunk while staying as a guest in his ex-girlfriend's Brockville, Ont., home in 2010, began cooking with oil in a frying pan with the burner set to high, then went out to Tim Hortons for 20 minutes. The house was destroyed by fire. Paul Tatton was charged with arson, punishable by up to 14 years in prison.

The case turned on the issue of whether prosecutors had to prove that Mr. Tatton had the specific intent to burn down the house, or whether it was enough to prove a more general intent simply to start cooking and leave. That more general intent is much easier to prove, and makes it harder for people who are drunk to claim their drinking as an excuse for the harm they caused, whether in arson or other crimes. Courts have made conflicting rulings for decades on what level of intent needed to be proved in cases of arson.

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"Alcohol habitually plays a role in crimes involving violent or unruly conduct and in crimes involving damage to property," the court said in a unanimous ruling written by Justice Michael Moldaver. "Although there are exceptions to this general proposition, the prevalence of alcohol in these crimes means that there are likely to be strong policy reasons militating against an intoxication based defence."

It was the third tough-on-crime ruling in two weeks from a court that has often taken issue with the Conservative government's tough crime legislation. Two weeks ago, the court moved to ensure adults cannot exploit the apparent consent of young people in order to make child pornography for themselves. It also refused to strike down convictions in cases involving aboriginal offenders when the government had tried and failed to find a representative number of aboriginal jurors to hear the case.

The Criminal Lawyers' Association, which intervened in the case, had argued that declaring arson a crime of general intent would mean someone who falls asleep while smoking a cigarette, thereby causing a fire, could be punished as an arsonist.

"What makes causing a fire a crime? It is the intention of the actor that converts what might otherwise be careless or innocent behaviour into a crime for which the person may be sentenced to 14 years imprisonment," the CLA said in a document filed with the court.

By a 2-1 majority, the Ontario Court of Appeal had said the Crown needed to prove Mr. Tatton specially intended to burn down the house. It upheld a trial judge's ruling that the fire was an accident, influenced by Mr. Tatton's inebriation. The Ontario government called that ruling "bad criminal law policy." (It also produced as evidence a note written an hour before the fire, in which Mr. Tatton threatened to set his ex-girlfriend's house on fire.) The Supreme Court said that in arson cases, prosecutors need only show "general intent," which is easier to prove. "It is difficult to see how intoxication short of automatism would prevent an accused from foreseeing the risk of causing damage to someone else's property by fire," Justice Moldaver wrote.

The court ordered a new trial for Mr. Tatton, saying that the trial judge had wrongly believed the man's intoxication was relevant. It said, however, that courts need to take into account all the circumstances of how a fire is caused, including whether it was accidental, negligent, reckless or intentional.

There was evidence at Mr. Tatton's trial that he was an alcoholic who would sometimes fall asleep while cooking, waking to find the house filled with smoke. His defence was that he thought he set the burner on "low."

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The broader issue at stake – of whether the Crown has to prove general intent or specific intent for arson and other crimes – is not spelled out in the Criminal Code. The court has ruled in previous cases that sexual assault is a general intent offence (drunkenness is not an excuse) but robbery and murder are specific intent offences (the accused has to intend the consequences, not simply intend, for instance, to strike someone over the head). Justice Moldaver said that Parliament should spell out which laws are general intent and which laws are not.

In legal terms, the issue is whether the crime of arson is one of general or specific intent. Specific intent requires the prosecution to show that the accused intended to create a fire; general intent is easier to prosecute because the Crown needs only to show that an accused person caused the fire.

The issue comes up in a wide range of crimes, the Ontario Attorney General said in a written argument filed with the court.

"This is an important question. Drunk people have been known to start fires, so the improper characterization of arson as a crime of specific intent will affect other arson prosecutions. And the reasoning of the majority of the Court of Appeal would arguably also lead to improperly characterizing many offences other than arson as crimes of specific intent to which an accused person's voluntary intoxication is admissible and relevant."