Almost exactly a century ago, there was a great scandal in British public life which created, according to GK Chesterton, as great a shift in public opinion as the First World War; indeed, he implied that the kind of thinking behind it precipitated that war.

It was the Marconi scandal. It receives nothing like the attention of other 20th-century crises such as the Wall Street Crash or the Bay of Pigs fiasco, but then Chesterton prophesied as much, writing that it was "probable that centuries will pass before it is seen clearly and in its right perspective". We needn't go into the details here (you can look them up), nor even ask whether Chesterton and his allies in the press were right about the accusations of insider trading they made against Government ministers; the thing to note is that the scandal was, at root, about probity in public life.

Plus ca change. Chesterton concluded (in his autobiography) that "Parliament had come to mean only a secret government by the rich". This insight has long been one of the tenets of that influential school of philosophy known as the saloon bar but, like so many of GKC's warnings, each passing day seems to make its central truth more evident.

It would be wrong, before the proper inquiries are concluded, to prejudge the past week's claims about the MP Patrick Mercer, who has resigned the Conservative whip, and three members of the House of Lords (an Ulster Unionist and two Labour peers) who have been suspended by their parties, all of whom are alleged to have offered to help commercial interests in exchange for payment. It may be that they have all, strictly speaking, abided by the rules on declaring Parliamentary interests, as they maintain.

Even if it turns out that the press have misrepresented these particular individuals, or seized on the wrong folk, however, there is still an issue to be addressed here. The Prime Minister anticipated as much before the last election, when he predicted that lobbying would become – as MPs expenses had previously become – the next issue to be investigated by the media, and a cause of public outrage.

Mr Cameron was right about that, but the reason that – even though he could see the train coming – he has done nothing about it may be that it is difficult to see what can be done in the way of rules and regulations.

There's nothing intrinsically wrong about trying to influence parliamentarians; indeed, they are there precisely for that purpose, and commercial enterprises, trades unions, charities and other special interest groups have as much right to try to make their case to them as individual citizens.

Nor is there anything undesirable about MPs, MEPs and MSPs having outside interests. Not only should they have the same right as the rest of us to take up any work or cause which interests them, but it is usually a good thing if they do. The most dangerous, unrepresentative and out-of-touch person is the professional politician with little experience of the real world – which, I hardly need add, is not, for most of us, the research unit of a union, the government relations department of a multinational, or a policy think-tank.

If an elected representative is convinced of the merits of any cause (no matter what it is) and is keen to promote it, it's hardly surprising that similarly interested parties outside Parliament will attempt to enlist his or her support. But it is important to distinguish between the two things. To take entirely random, hypothetical, examples, defending the civil liberties of smokers, or the right of workers to organise, is not necessarily the same thing as promoting the commercial agenda of British American Tobacco, or the political agenda of Unison. Even then, there may be nothing wrong with the second course of action, as long as any financial interests are transparent to the electorate.

All sorts of things may be in MPs' personal interests. The most obvious is the readiness (no matter the logic of their underlying political convictions) of almost all of them to argue for subsidy or support for businesses or public bodies in their own constituency. But any other external connection, whether it be the directorship of a business or the backing of a trade union, may be just as valid; may, indeed, be actively useful, by providing them with insights or expertise that help avoid poor legislation.

But if prohibiting outside interests altogether would make politics very much worse, as well as being obviously illiberal and illegal, it doesn't alter the fact that almost everyone can see that there are (at least potentially) many cases where elected representatives abuse their position for their own enrichment or that of their cronies.

That is why there are rules about declarations of interest – and, in fact, the rules ought to be perfectly adequate for the purpose, because if we know that any politician has any personal interest in a policy he or she is advancing, we can make our own judgment about whether that influence is proper or not.

But it is also why the problem persists. The very existence of a set of rules prohibiting some kinds of behaviour – as the current row over tax avoidance by large multinational companies illustrates – is also taken as licence to do anything which is not a technical breach of the rules.

In the Marconi case, Lloyd George and other ministers carefully found a form of words which avoided lying, but which had the express intention of deceiving the public. Most of us would support Chesterton's view that this weaselly, legalistic technique is, if anything, actually worse than outright lies.

And herein lies the problem with lobbying, as it did with MPs' expenses, and as it does still with bankers' bonuses and corporations' tax strategies. The more the rules are laid down and defined to allow individuals to use them as a substitute for taking moral responsibility, the more they are likely to see their behaviour as blameless, even if everyone else believes that it is indefensible.

The politician who argues for cuts in fuel duty, while owning shares in an oil company, or for raising levels of legal aid when he or she is a lawyer, or for nationalising an industry when financially backed by its chief union may be acting neutrally, judiciously, and in accordance with his or her beliefs. Or he or she may be acting corruptly, or even admirably.

It cannot be determined by the regulations alone, which is why tightening them won't by itself solve the problem. It depends, case by case, on moral judgment – theirs and ours – as well as the strict wording of the rules.

To ignore that is like a fishmonger saying that a rotting haddock is still perfectly delicious, because its use-by date isn't until tomorrow – it does nothing to disguise the bad smell from the customer.

The trouble with that kind of stink is the fishmonger becomes immune to it. The insidious aspect of self-seeking and promoting vested interests is that it eventually robs politicians of all their moral sense to the point where, having sold their own souls, they sell the rest of us down the river, too.