George H.W. Bush's ''no new taxes'' in 1988, for example. Likewise for John Howard's 1995 doorstop promise: ''There's no way that a GST will ever be part of our policy.'' Julia Gillard's ''no carbon tax under the government I lead'' in 2010 remains a particularly clumsy bit of spin. Each government did the deed eventually.

Most recently, Opposition Leader Tony Abbott promised to repeal the carbon tax. If he is elected, he will probably break this promise. If so, perhaps it was never a ''core commitment'', to quote his mentor John Howard.

And this is only tax: the graveyard of dead pledges is heaped with promises about war, religion, wages and the rest. Some of this is due to incompetence, some to immorality. Either way, we don't expect what Abbott called ''the gospel truth'' from leaders.

Sometimes this is straightforward deception. At least since Plato suggested his ''noble lie'' over two millennia ago, we have recognised the role of deliberate falsehoods in public life. ''Men are so simple, and so much creatures of circumstance,'' wrote Machiavelli in The Prince, ''that the deceiver will always find someone ready to be deceived''. Plus ça change.

But in the last decades, polls have noted a decline in public trust - not in one another, paramedics, doctors, the police, or even bankers, but in governments. And busted pledges, to say nothing of straightforward lies, are undoubtedly part of this: politicians do not keep their word, and voters know it.

Importantly, recognising this fact does not require acceptance or rejection of specific policies. F. D. Roosevelt, for example, had good reasons to abandon America's official policy of neutrality. I support Gillard's carbon tax, even though its global atmospheric consequences are small. Broken promises can, in other words, still provide good policy.

But the price for fickle representatives is a high one: frustration, disengagement, apathy and lowered aspirations. Why do they do it? There are many reasons why politicians break their promises, including cynicism and incompetence.

Some, in a typically Machiavellian move, simply lie to get votes, while others lack the wherewithal, clout or rhetorical nous to pass policy.

These cases certainly contribute to the idea that elected officials are charlatans or bunglers.

But there remain many cases where a leader's pledges, though given lucidly and sincerely, cannot stand. This was behind John Howard's defence of political somersaults, in 1998.

''There is absolutely nothing wrong … with stating a position on something, honouring that during the lifetime of the term … and then, if you alter your position and you go to the public again with the altered position, having a right to implement it after that election.'' At first, this seems reasonable. Politicians are not alone in this mismatch between rhetoric and reality. Most employees in private industry also have to work with imperfect reality: the world changes, and so do alliances and moods.

The world is complex, and we are biased and capricious creatures.

Surely promises can be broken, or just stretched a little?

In a word: no.

These circumstances are exactly why we make promises. They are not pledges of fleeting feeling. They are not claims to mastery over the universe.

They are simply public agreements that we will follow through on our word.

And if we can't keep a promise, we don't make it - this is what we teach our kids. And we do this partly because of private interest: if I keep my word with others, says the common wisdom, others will keep their word with me. To maintain the rudiments of civilisation, we need not be saints: just sincere and reliable.

This is doubly important for politicians. Precisely because so many voters are privative and partial, politicians are deputised to work in the public interest. Their personal desires and anxieties are far less important than their pledges. By giving their word, they add a stitch to the common fabric of the country.

Without this basic trust, representative democracy frays. Its familiarity makes it seem less ridiculous than it is.

But imagine if our spouses, companions or colleagues were as fickle as politicians.

Marriage, friendship and business would simply cease. Yes, many still vote for these candidates, but this is often because of limited choice. With the exception of a handful of true believers, Australians now expect betrayal on election promises from both major parties.

In a marvellous irony, the generation of politicians that most champions competition has enjoyed a two-party duopoly, which encourages neither party to keep their word.

There is no magic pill to restore trust.

But voters' diminished party identification suggests one obvious response: a refusal to vote for candidates and parties with so many pledges, but so little promise.

Dr Damon Young is a philosopher and author. His next book, Philosophy in the Garden, will be published in December 2012.