What constitutes guilt? Does knowing lives are being taken without actually ending them yourself create a criminal responsibility?
They are questions which go to the very heart of a historic court case beginning in Germany today.
Seventy years after the end of the Second World War, a 94-year-old man is going on trial accused of being an accessory to the murder of 300,000 people in the Auschwitz concentration camp. That man is Oskar Groning.
At the age of 21, back in 1942, the young SS officer was sent to Poland committed to the Nazi belief that Judaism had to be destroyed.
Over two years he became the bookkeeper of Auschwitz. He counted, ordered and logged the money stolen from those taken from their lives to their deaths.
Each day his work would demonstrate the global attack on the Jews - as he tallied up the Polish Zlotys, French Francs, Greek Drachma, Italian Lira and Dutch Gilders.
But Oskar Groning was not just hidden in an office aware only of his ledgers. He also stood guard as Auschwitz's desperate human cargo was unloaded, going through the belongings of those who would be exterminated for their religion.
He says he didn't kill anyone, he didn't pour the Zyklon B into the chambers and he didn't burn the piles of dead.
But he knew it was going on, he saw it happen and he accepted mass murder as part of the routine.
At the end of the war he became a British prisoner then returned to Germany to rebuild his life. He married, he told his wife never to ask about his role in the SS and went on to have two sons, build a career as an accountant and enjoy stamp collecting.
He may have got away at that point - only 50 out of an estimated 6,500 SS guards have ever been convicted.
But Oskar Groning's past could not be so easily obscured. In the end, it was to challenge the holocaust deniers that he spoke out. He says he was not an active perpetrator, just a small cog in the gears but he was a part of a very real atrocity.
What level of responsibility he bears will be decided by a court in the German town of Luneberg. It is where the first Nazi war trial took place and given the passage of time it is likely this will be the last.
It is a final judicial chance for Germany to wrestle with a history that continues to cast long shadows and a final chance to determine if being present as hundreds of thousands were murdered constitutes criminal responsibility.
The question of moral responsibility may not be so difficult to determine. For all his frailty Oskar Groning remains a disturbing example of what humans are capable of.