Blame is the easy bit. You can look at the death of Baha Mousa and talk of how an entire nation has been shamed by the first member of the British armed forces ever to be convicted of a war crime. But there is a more interesting question.

It was framed by Garry Reader, the private with the 1st Battalion the Queen's Lancashire Regiment, who gave evidence against his fellows at the Gage inquiry into the conduct of British troops in Iraq. It does not look outward, like shame. It looks inward, like guilt. How do you live with having been part of an event like that?

The contrite soldier gave an interview in which he said: "There ain't a day that goes by without I don't think about it, or a night that goes by that he's never on my mind. I've got to live with it for the rest of my life... that's nothing in comparison to what his family, his kids, have gone through... but it affects me and my family. I feel like I can't move on with my life."

Reader is plagued by "what if" questions. "What if I'd walked in five minutes before? What if I did something wrong in the CPR (cardiopulmonary resuscitation)?" This is the coinage of both guilt and grief. "History is not merely what happened," the historian Hugh Trevor-Roper once said. "It is what happened in the context of what might have happened."

Historians have played this as a parlour game. What if the dinosaurs had survived? What if Socrates, the founder of Western thought, had died in the Peloponnesian War? What if the Muslim scholar who built a flying machine in AD711 had given it a tail and Islamic planes had conquered the world? What if the Chinese not the Europeans had discovered America? What if Charles I had beaten Cromwell? What if the British had defeated the Americans in their War of Independence? What if Lincoln had not freed the slaves? What if Napoleon had won the Battle of Waterloo? What if the Nazis had won the Second World War? What if Kennedy had not been assassinated in 1963 or Gorbachev been in charge in Russia in 1989?

There is more to this than an intellectual conceit to ram home that the present is not the only possible destination of the past. The German philosopher Hans Vaihinger in 1911 published a book called Philosophy of As If. It said that human beings need willingly to accept falsehoods to live successfully in an irrational world. Not "what if" but "as if". Just as we must proceed assuming that the material world exists – though logic can prove only that it is a perception – so we must act as if ethical certainty were possible even if it is not. Thus our inward selves turn outward.

Free will is a classic example. A determinist worldview persuades many philosophers, but in practice we need to proceed as though human choice mattered. Courts of law would be otherwise pointless. So would committees of inquiry into military violence.

What that teaches us is that Garry Reader should have started asking "what if" a good deal earlier than he did. So should his battalion commander Colonel Jorge Mendonca, who was cleared of knowledge of the beatings, but told he should have known what was going on. So should Reader's unit commander, Lieutenant Craig Rodgers, who should have acted "as if" Iraqi lives were morally equivalent to British ones, even if he did not feel that at a time when his fellows were suddenly being killed in numbers in Basra. Had he acted earlier, the Gage report concluded, Baha Mousa would "almost certainly have survived". What if he had? So many lives would have been better.