“Ignorance about the fate of transferred prisoners is surely a case of vincible ignorance,” Cormie told The Catholic Register by e-mail.

Canadians in Afghanistan at the very least had a positive duty to investigate credible claims of torture, he said.

“On the basis of the most general knowledge of the Afghan history, the general operating presumption should have been, not the good treatment of prisoners, but the opposite — the regular torture of prisoners,” said Cormie.

Plausible deniability doesn’t play in Catholic moral theology, said Cormie. He cites paragraph 1859 of the Catechism of the Catholic Church on the nature of mortal sin: “Feigned ignorance and hardness of heart do not diminish, but rather increase, the voluntary character of a sin.”

“If you do things that avoid direct complicity, but you know all along that indirectly (you are helping), you are complicit still,” said Regis College professor of moral theology John Berkman.

A scheme of plausible deniability in which people conspire to make it appear they did not know and therefore are not responsible for the actions of others makes things even worse, according to Berkman.

“You are not only involved in torture and all that, you are also involved in deception,” he said.

The positive duty to investigate derives in part from the seriousness of the sin. Torture — along with abortion, genocide, homicide, euthanasia, human trafficking, subhuman conditions of work and unjust deportation — was named by Pope John Paul II as intrinsically evil (intrinsece malum), “acts which per se and in themselves, independently of circumstances, are always seriously wrong by reason of their object” (Veritatis Splendor, paragraph 80).

“If you have any serious doubts you should never be handing them over in the first place. That’s the Catholic position,” said Berkman. “They should have investigated. They had an ethical obligation to do that.”

Berkman does not believe that Catholic moral theology, with its categories of venal and mortal sin, is the best way to engage the political and policy debate over the prisoner abuse scandal.