Ah, Mr Burnham, come in, take a seat. Mr Streeting, good to see you. We’ll also be interviewing Mr Starmer, then Ms Rayner; Mr Farage and Mr Polanski come in this afternoon. So, this prime minister job: what are you in it for?

That’s how I would do it. The “what are you in it for?” question gets to the heart of personal ambition. Of course we all tell prospective employers that we are hard-working, able, conscientious and ambitious. But that last claim, in particular, needs to be followed up and tested a bit. Ambitious for what? Ambitious for whom?

This period in Westminster is being fuelled by naked ambition: either the desire to stay in office or the determination to get to the top. Politics – sometimes unfairly disparaged as “show business for ugly people” – does not always reveal humanity at its best. One way of looking at the disastrous catalogue of bad government we have endured for over a decade now is as an exercise in bad ambition running riot. As the author of Fair or Foul – The Lady Macbeth Guide to Ambition, able to view it all through that Shakespearean lens, I can say how that goes.

The rivals will use respectable-sounding language, invoking duty, public service and even honour. Some leading figures may have convinced themselves that their motives are essentially worthy. But, in truth, Westminster is a battlefield where power is sought and exercised.

Sometimes this truth is acknowledged. When he appeared on Desert Island Discs over 20 years ago, Boris Johnson was candid enough to admit what drove him on. “My silicon chip, my ambition silicon chip, has been programmed to try to scrabble up this cursus honorum, this ladder of things,” he said. One of Johnson’s former employers, Max Hastings, once gave the former prime minister a less-than-glowing character reference along similar lines. “He is a fundamentally weak man, save in his personal ambition,” he said.

Keir Starmer is in trouble partly because his ambition to succeed does not seem to be rooted in a specific political project. He wanted to be prime minister. But why? Wes Streeting’s damning critique, set out in his resignation letter – “Where we need vision, we have a vacuum. Where we need direction, we have drift” – resonated. It would have helped if Starmer appeared to have some enthusiasm for the business of politics. But an angry electorate looks on and does not understand what, if any, overriding purpose he has.

If you are a skilful operator, and luck is with you, it might be possible to survive at the top of politics for a while without a bigger goal than the ambition to hold on to the job. David Cameron once said he wanted to be prime minister because he thought he would be “good at it”. But this was not a very good answer to the “what are you in it for?” question. Eventually, his luck ran out. And now, after his election win less than two years ago, which produced a massive majority (albeit on a modest vote share), Starmer’s luck has run out, too.

It’s bad news for all of us, as this experience has done nothing to reduce the cynicism of the electorate. The much-heard vox pop response of “they are all the same/all in it for themselves” has become the soundtrack of our times. This is useful only for the populist parties – and especially Reform. It becomes easier for Nigel Farage to try to laugh off accusations of dodgy practice if he can suggest he is no different to or no worse than anybody else, just more blatant (and shameless) about it. We risk ending up in a bleak world in which everyone’s motives are questioned, and no ambition can be good or pure.

Macbeth is a warning about ambition gone wrong. When Lady Macbeth says of her husband, “thou wouldst be great, art not without ambition, but without the illness should attend it”, she makes a disturbing point about the depths to which we can sink to fulfil our dreams. She understands what the worst of us are capable of.

But ambition is not wicked. It does not have to be violent or selfish. We can be ambitious for the greater good, to work towards achieving great causes. We can be ambitious simply to lead a good life. We need ambitious people to make scientific discoveries, launch new businesses, solve urgent problems, provide good leadership.

Don’t take my word for it. In a commencement speech to recent graduates at Arizona State University recently, the actor Harrison Ford said this: “Whatever talent or ambition you have, find some way to put it to work. Build something that didn’t exist yesterday. Stand up for someone who can’t stand up for themselves. Bring people together who weren’t talking before. That’s leadership. That’s what moves the needle.”

So, look at the candidates to be PM. Look, perhaps, at yourself – what are you in it for?

• Stefan Stern is the author of Fair or Foul – the Lady Macbeth Guide to Ambition