James O'Brien tweeted yesterday:

McGuinness was both a murderous terrorist & a powerful force for peace. He changed. Our furiously binary zeitgeist can't compute such change.

I want to expand on this, because it tells us something about the nature of politics generally.

I suspect McGuinness was a force for peace in part precisely because he was a murderous terrorist. Because his credentials as an IRA man were so strong, he could persuade hardline terrorists to give up violence in a way that more moderate republicans could not. Granted, his commitment to peace might, as Padraig says, have been "tactical rather than principled". And he might have adopted it from a position of weakness: by the 90s, the IRA was so chocka with MI5 agents that it resembled something from a G.K.Chesterton novel. But peace is peace.

There are many examples in politics of men changing events because their previous commitments gave them credibility with potential opponents of that change. We've even got a name for it: "Nixon goes to China". Nixon's impeccable anti-Communist credentials meant that he could begin détente in a way that more liberal men couldn't because of the fear of being labelled soft on communism. Similarly, Tony Blair was able to abandon Labour's Clause IV in part because he had the support of John Prescott, a man whose deeper roots in the party gave him more influence over Labour traditionalists than Blair alone could enjoy.

Perhaps a closer analogy with McGuinness's change, however, is the role Lyndon Johnson played in the passage of the Civil Rights Act. LBJ was a racist - certainly by today's standards and perhaps even by those of his time. Such attitudes, however, gave him influence with southern segregationists that Kennedy - who had proposed the Act - never had. LBJ thus managed to force the Act through Congress whereas Kennedy failed. JFK might have been more acceptable to decent people, but LBJ did the job.

LBJ was both a racist and an advancer of blacks' rights, just as McGuinness was both a murderer and a force for peace. And both men were one because they were the other.

George Bernard Shaw famously said that "all progress depends upon the unreasonable man*." This might be an exaggeration, but one way in which it is true is that the unreasonable man can persuade other unreasonable men in a way that moderates cannot.

It's in this context that James is right to decry our simplistic "binary zeitgeist". Many people think of politics as a low-grade morality play in which good people - people like us, naturally, because we lack the faculties of self-criticism - oppose bad people. But it isn't always so. McGuinness and Johnson show that "bad" people can sometimes do good things, perhaps even for bad motives. And the converse can also be true: good people can do bad things. The social sciences are often complex emergent processes: outcomes aren't always reducible to individuals' intentions.

Personally, I'd like to see less moral posturing and tribalism in politics and more inquiry into how to build structures that increase the chances of bad people doing good things and lessen the chances of good ones doing bad things. But this is a forlorn hope.

* It's sort of fitting that Chuck Berry should have died in the same week as Martin McGuinness, as he - in his very different way - is another example of how dubious characters can do great things.