Many people will agree with the old saying that "all's fair in love and war". Others would add "and politics" - a view that certainly seems to be taken by some politicians.

I have never accepted the notion that the ordinary principles of decent behaviour should be suspended when it comes to politics. Someone who lies and cheats in politics is no less a liar and a cheat. Indeed, in politics it is arguably an even greater offence since it is brazenly committed by those who owe a duty of trustworthiness to the public.

But there is no shortage of politicians ready to behave irresponsibly and dishonestly for the sake of personal or political advantage. The latest, very committed recruit to these disreputable ranks is the US Republican presidential hopeful, Donald Trump.

Mr Trump has distinguished himself - if that is the right phrase - with snap judgments in the most extreme terms on issues that surely require more careful consideration.

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Mr Trump seems to have calculated that the only thing that matters is that he should stay in the headlines. He may insult women and virtually every kind of foreigner; he may propose outlandish "solutions" to complex problems, such as building a wall across the whole US-Mexico border to keep out Mexican "rapists", or banning entry to the US of anyone who could be described as Muslim; his concern is not to make sense but to make news.

This leaves us with a dilemma. Is he to be taken seriously, or is he just a buffoon? The opinion polls suggest, for the time being at least, that he needs to be taken seriously. But that raises a different question. Is he just a demagogue prepared to say anything or, even more worryingly, does he really believe what he says?

It is a measure of how self-absorbed a large chunk of the American public is, and of how little they are aware of the world beyond their borders, that they seem completely unaware of the dire picture painted of their public life by the possibility that Mr Trump could be elected to the presidency. The rest of the world can only look on in disbelief at the prospect of the free world being led by such a loose cannon.

Before we become too self-righteous, however, we should acknowledge that our own public life is not immune from political leaders prepared to act - even if on a substantially smaller stage than Mr Trump's - in a way that falls short of the standards we might reasonably expect. We might, sadly, not be overly surprised that this is the case - but what is certainly disappointing is that when such a lapse is brought to the attention of the public, and even more so of the media, the consequences for the wrongdoer are minimal.

This certainly seems to have been the case with the return to Cabinet of Judith Collins. The response has been, in general, a tolerably warm welcome for the return of someone seen as a repentant sinner.

The "political comeback of the year" has been portrayed as a matter for congratulation. Her supposedly admirable qualities as a politician - exemplified by her cultivation of the sobriquet "Crusher" - have been celebrated. There is even renewed talk of the possibilities of her ascending to even higher office.

This is surprising, not just because of Ms Collins' manifest errors of judgment and policy as a Minister (just think of Serco's contract to run Mt Eden prison), but also because we know quite a lot about her approach to politics.

Most people will not have read Dirty Politics but the journalists certainly will have done. We don't need to rely on hearsay or reportage to judge Ms Collins, because we can turn (for good or ill) to the words she herself used in email correspondence with Cameron Slater and others.

No one reading that correspondence can be in any doubt about the distorted view of politics, and political life, the returning Cabinet Minister holds. A political career is, according to this view, not a matter of principle and service, but of bitter and vindictive in-fighting, requiring dirty tricks, ambushes and plotting, all expressed in the language of the gutter.

Before we cast stones at other people's glasshouses, should we not expect those who report on our own public life to provide us with the whole picture?