Peter Mandelson backs winners. If as we learnt (to gasps of incredulity) yesterday, Gordon Brown's old enemy is now to sit at the Prime Minister's side in Cabinet, then some of us will contemplate a choice between only two available conclusions. Either Mr Mandelson now sees Mr Brown as a potential winner or he will in the end betray him.

Nobody should rule out the second possibility. “A fighter not a quitter,” Mr Mandelson once so memorably called himself, but why pose these options as mutually exclusive? In his time Mr Mandelson has been both: he has quit and lived to fight another day; and he has fought and lived to quit another day. He fights again. He may quit again.

Mr Brown may be calculating that, having resigned from a Labour Cabinet twice already, and then resigned from British politics altogether, Mr Mandelson dare not risk that fourth “I QUIT” headline. I wouldn't count on it. But the embattled Prime Minister should be safe until next summer, and in these troubled days for Downing Street we have to remember that Brown years, like dog years, imply that a month's security in real time is worth about seven.

But what a roller coaster. At first they told us that this autumn reshuffle might be a limited affair. Then more recently they told us it was going to be big. As late as Monday this week Gordon Brown's Brigade of Unattributables were still insisting it was going to be big. Then on Wednesday a new message went out: it was going to be small.

So when we switched on our radios yesterday to hear that it had arrived - and was spectacular - we can be forgiven both a certain confusion and the media hoopla that always accompanies the thrill of the unexpected. The twists and turns that this story has taken are evidence either of a devilishly clever media communications strategy or of Mr Brown's complete inability to make up his mind until the last moment.

Who cares which. He has made it up now. We have the reshuffle and as I write we have commentators from outside and within the Parliamentary Labour Party running around like startled ants, voices united in the age-old cry: “What does this mean?”

Very possibly nothing. It takes, at such times, a determinedly level head to hold on to the age-old truth that questions such as this usually deserve the response that it won't make much difference in the end. It certainly won't save the Prime Minister in the end.

On Mr Brown's part the reshuffle can be seen both as an act of panic and of calculation. It is not irrational for a man losing his footing to grab at a cactus for support; but the cactus is still a cactus. It is even worth entertaining the possibility that the Prime Minister's first thought was to get himself another really competent and effective Business Secretary - and that it was Mr Mandelson's (often overlooked) high reputation as a sound administrator, fine dispatch-box performer, and assured commander of his brief, that recommended him most. Whatever else this appointment may mean, it will reassure the City and bring to the front bench in the Lords a wily and dogged advocate of new Labour's Blairite compact with the rich and the wealth creators.

And how about from Mr Mandelson's perspective? For him this will not have been a difficult offer to accept. We should examine it with the same hard-headedness as Mr Mandelson himself will have done as he heard from the Prime Minister this week. He faced unemployment next year when his term as EU Trade Commissioner will be up. He was widely thought to envisage a career thereafter in a top-flight job in business, industry or finance.

But if 2009 is to be a year of recession, with the bodies of company chairmen raining down from high storeys in Canary Wharf, the moment would hardly be ideal for job applications from a former politician intimately associated with a Labour Government limping towards imminent election defeat. Mr Mandelson is only 54. He will feel he has at least one and maybe two more big jobs in him. “Well, what the hell?” will have been his first and instinctive response. Whether or not his second thought was, “I can always bail out later”, it has certainly occurred to me.

We see now what Mr Brown meant when he told his party in Manchester that government was for people with experience. In come the cockroach people - men and women who keep getting stamped on but who somehow never die. In comes Margaret Beckett - no novice she - who first became a minister under Harold Wilson when I was still at university, and whose most recent experience was to be sacked as Foreign Secretary. Sideways goes a politician, Geoff Hoon, whom experience has taught not to be memorable, and whose amiable anonymity has proved a trusty friend - relieved just to survive. Back (as Chief Whip) comes Nick Brown, whose experience was to be junked by Tony Blair.

And in comes Peter Mandelson, of whose restless intelligence I have so high an opinion, and the news media such exciting memories, that few of us can see him plodding quietly into electoral oblivion. Whatever else he may do, Mr Mandelson fascinates.

But so do scorpions. Do you need to be reminded of the fable of the scorpion and the frog - the story of a stinging insect desirous of crossing a river, and hitching a ride on the back of a frog? “But you're a scorpion,” says the frog, “you sting”.

“Normally, yes,” replies the scorpion with both apparent and perhaps real sincerity, “but why would I do that when it would sink us both, drowning me too?”

Compelled by this logic the frog obliges. Halfway over the water, the scorpion stings. The frog's last words are: “In the name of God, why?” The scorpion's last words are: “Because it is in my nature.”

Some will think it would be in Peter Mandelson's nature, too. I, however - though I see no reason why my sharp-tailed and quick-reacting old adversary should think 18 months too short a time in which both to enter and to exit from a Labour Cabinet - do not assume that he would be extinguished by his own sting.

At present, surely, Mr Mandelson is plotting no such thing. But I strongly suspect that sooner or later the idea will occur to him. A general election whose aftermath sees Gordon Brown washed lifeless on to the riverbank will see Peter Mandelson still swimming. A man whom some top Tories have jokingly called “The Master”, and who has appeared neither displeased nor repelled by the appellation but coyly attentive, displays an extraordinary ability to swim.

So you're right, Prime Minister: experience counts for a lot. Remind me again, Peter, what's been your experience of Gordon Brown and his henchmen? Remind us, too,

Mr Brown, what experience may have taught you about Mr Mandelson? Myself, I've been taught that he will not go gentle, or without a splash, or forgettably, into that good night.

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