At McDiarmid Park, Perth, on Saturday afternoon I took part in a very warm minute's applause to celebrate the life and work of Nelson Mandela.

Everyone in the ground, of whatever age, appeared to be enthusiastically involved. Similar manifestations of genuine affection and respect have apparently been taking place right across the globe.

Later on, during the more boring passages in the football, I reflected on this unlikely outpouring of an almost tangible veneration for a distant, only vaguely apprehended figure. Why should a few thousand people assembled on the edge of a provincial Scottish town so gladly endorse the memory of a man whose achievements had not directly affected them at all? Remarkably few world leaders come close to achieving this kind of resonance, this almost sanctified status. And Mandela was hardly a world leader anyway, more a very important regional one.

The great global expression of reverence for this fine man maybe reflects how very few genuine political heroes (as opposed to concocted showbiz ones) the billons of people on our planet really possess.

How many comparable figures, even beyond politics, have there been in Mandela's own lifetime? It's easy enough to suggest a few, a very few: one or two Popes, perhaps, and maybe Martin Luther King, Mother Teresa of Calcutta, the two Ghandis (Mahatma and Indira), Ataturk …and Mikhail Gorbachev, and just possibly Mao Tse Tung, but very soon the list begins to peter out. And even some of these people were noted almost as much for their crimes as for their inspirational leadership.

What is most problematic is how to measure achievement. Nelson Mandela fought evil in a courageous, dignified and conciliatory way, but the evil he eventually triumphed over - though appalling - was not on a vast scale.

Nazism was by far the greatest organised evil of the last century, probably the greatest evil the world has ever known. It took a colossal extended effort to crush it. Winston Churchill played his part, of course; from early on he understood as few others did the unique menace and wickedness of Nazism. But if one individual defeated Nazism it was Joe Stalin.

And yet if anybody suggested that Stalin should take his place with Nelson Mandela they'd be mocked mercilessly.

Stalin was an utterly ruthless war leader. The point is that only a man of callous and almost inhuman drive could have presided over the defeat of Adolf Hitler and his vast war machine. The Russian people's sacrifice in their heroically sustained fight against Nazism was exceptional, not only in terms of the bloodcount, which was on a scale that was almost unthinkable, but also in terms of the sheer scope of the military and industrial effort. This was understood by Stalin's allies like Churchill and Roosevelt, and later, President Harry Truman, who actually claimed that he liked "Uncle Joe" Stalin (few others did).

Stalin was responsible for atrocity upon atrocity, for organised barbarism that was an extended insult to civilisation. And yet his achievement, in taking the lead in defeating Nazism, was probably greater than that of any other single human being in the twentieth century. This is the paradox. It is about using and orchestrating evil to defeat an infinitely worse evil. Looking back on the 1940s, it would seem that civilisation itself cannot muster the forces to defeat something as horrific as Nazism. In the summer of 1941 most of mainland Europe was in the hands of the Nazis or their allies. At that point Russia, too, was of course an ally of the Germans. It was Hitler's idiocy, in attacking Russia, that immediately turned Russia from friend to foe. And what a superb foe Russia turned out to be.

We must remember that even so long after the eventual military defeat of the Nazis, their perverted racist ideas have not been totally obliterated. That's where we should come back to Mandela. Maybe, just maybe, his example will help to prevent the emergence of a huge organised evil as hideous as Nazism in this relatively young century. We must all hope so.