Wait. Wait. Wait. Wait. Wait. Wait. Wait. Wait. Wait. And then? Keep waiting.

That is my advice to David Cameron on how to oppose Gordon Brown. Useful, eh? I’ll tell you how I got there.

I began with this question – who is David Miliband? Besides being Foreign Secretary, that is. What really drives him? What does he think? What will he do?

Only a few weeks ago, Mr Miliband was being touted as the Blairite challenger for leader of his party. Friends of Tony Blair sought to persuade him to stand against Mr Brown, but he balked when he couldn’t persuade unaligned figures like Hilary Benn and Alan Johnson to back him. Only the most factional Blairites urged him on.

But after his appointment to his new post, we were offered a different picture of Mr Miliband. He is a private critic of the Iraq war, apparently, and the only person to speak out in Cabinet against Mr Blair’s refusal to call for a ceasefire during the Lebanese war. Oh, and his critics call him a Eurofanatic. Then again, he talks fluently about the importance of promoting democratic forces in Iran, a neocon position, and is regarded as a hardliner on relations with Muslim extremists in the UK. He’s also personable and open, but more ideological than Mr Blair.

So is he a Blairite or a left thinker? The political son of new Labour or the actual son of a Marxist intellectual? When I put this question to a friend of his, the former Blair aide Matthew Taylor, he asked me if I knew how to classify people’s sexuality. Er, carry on, I replied nervously. It’s not as simple as dividing people into gays and straights, he said. You use three categories. Do you care about your appearance? Do you flirt with other men? Do you sleep with other men? So Tony Blair, for instance, is gay-gay-straight.

Mr Taylor then suggested a similar way of categorising Labour politicians. How intense and driven are? How tribal are you? What are your beliefs? Use this scheme and you get the perfect description of Mr Miliband. He is Blair-Blair-Brown.

Clever. Neat. But I think things are a bit more complicated than that. I think that the reason it is hard to work out who David Miliband is is that he isn’t sure himself. And he is not at all unusual in that.

Political beliefs are, to use social psychology terms I have employed before in this column, situational at least as much as they are dispositional. That is, they are determined at least as much by circumstances as by deeply held moral convictions.

By the end of his premiership, Tony Blair, once a member of CND, had become a military hawk and ally of a Republican president. And he looked back at his 1997 pledges and described them as “footling”. He was a different man in 2007, because he was in a different position. It wasn’t simply what he was saying that had changed. He had changed.

Look at the deputy leadership contest. Almost all the candidates shifted from the Blairite orthodoxy the moment they became more than simply members of Blair’s Cabinet. In a different situation they behaved differently.

And so it will be with David Miliband. He has spent his entire political life working for Tony Blair and Mr Blair has changed him profoundly. I remember being very struck back in 1994 in those early days with Mr Blair, watching as Mr Miliband took on the leader’s project as his own, his convictions gradually blending with his boss’s. But with Mr Blair’s departure his situation has changed utterly and he will change with it. He is bound to.

As will his colleagues. Gordon Brown appointed a new Cabinet with very much the same sort of people who were in the outgoing Cabinet. But it will be a vastly different body because Tony Blair isn’t in it. Without him, without his leadership, without the pressure he exerted on his colleagues to behave as he wanted, all those familiar faces around the table will start behaving in unfamiliar ways. Tony Blair is the elephant that’s no longer in the room.

Nor can you rely much for help on what Mr Brown has told us so far. He has talked of change, but his ministers have been hilariously incapable of explaining what that might involve. His foreign policy appointments have faced both ways. And his constitutional package, while good and solid stuff, is the sort of thing you do while you are thinking of something else to do. It’s all been politically skilful, but fantastically unrevealing.

Which leaves David Cameron with a problem. His party wants him to go on the attack. Commentators want him to provide a critique. And as the weeks turn into months, these calls will get louder. But he doesn’t know what he is providing a critique of, or what he is attacking.

So he has to wait. Early in his leadership Mr Cameron tried calling Mr Brown the “roadblock to reform”. On becoming PM, Mr Brown simply announced he would continue Mr Blair’s reforms. When Mr Cameron moved to saying that Mr Brown was the “no change” candidate, the new PM began using the word change every ten seconds. All these criticisms do is give Mr Brown easy ways to make Mr Cameron look silly.

They also make the Tories look unreasonable and ungenerous. The public think, understandably, that a new prime minister should be given a chance. And they’ll be unimpressed if they see Tories on television launching premature, partisan attacks.

This is not a minor point. It’s as important as policy. Watching the Tories respond to the Government is one of the main ways that voters encounter them. The pressure to go on the offensive will be strong. It is vital to Mr Cameron’s mission to present his party as moderate, attractive and public-spirited that it be resisted.

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