I tweeted last night, to the consternation of some, that David Miliband is one the the nicest blokes I've met. I did so for three reasons, other than that it's true.

First, to counter a couple of common cognitive biases. One is a form of the halo effect - the idea that because someone has bad politics, he is a bad person. This is just wrong. Sometimes (often) bad people do good things, and good ones bad things. The other is a form of outcome bias. Peter Oborne describes David as "greedy", but he never struck me as that at all. Just because someome is rich does not mean they are greedy. They might instead be just lucky, in the right place at the right time.

Secondly, and relatedly, I want to counter a common conception of politics, especially on the left. It's what I've called the Bonnie Tyler syndrome, of holding out for a hero - the idea that our political objectives can be achieved if only we could be governed by the right people. But this is not true. It ignores the power of capitalism to perpetuate itself, for example because it generates ideologies that favour inequality, and because capitalists' control over investment decisions gives them control over the economy. And it ignores the fact that even quite senior politicians have less influence than is attributed to them; as I've said, office doesn't just corrupt, it enslaves.

We pay too much attention to the character of politicians, and not enough to the power structures and ideologies that constrain them.

Remember - David did have radicalish instincts. In 1994, for example, he described inequality as "shocking" and pointed out that "capitalist economies contain basic inequalities of power."

Thirdly, I did not mean "nice" entirely as a compliment. One fault of agreeable people is their tendency to trust others too much, perhaps because they wrongly project their own decency onto them. I fear this tendency led David to become too trusting of Blair.

The message of David's career, then, is that good instincts, decency and intelligence are not enough for a politician to change society. It is an old joke that Ralph Miliband thought that socialism couldn't be achieved by parliamentary means, and his sons are proving him right. But there's an element of truth in this.