It takes an opinion poll to quantify our wish to have our cake and eat it. Although I think, incidentally, that the phrase ought to be "to eat our cake and have it". What we want is to enjoy the cake and still have it to look forward to. Anyway, as with cake, so with rail fares. A YouGov poll at the weekend found that 87 per cent of people say that the planned increase in rail fares is "unreasonable".

Unusually, though, YouGov then asked another question. "If you had to choose, which one of the following would you prefer – an increase in rail fares of 6.2 per cent, or an increase in the basic rate of tax from 20p to 21p?" That forced a reassessment. Suddenly, the "unreasonable" fare rises became the preferred option of the largest number, with 39 per cent preferring higher fares and only 32 per cent sticking with the implication of their answer to the first question, and saying that they would be happy to pay, or have other people pay, higher taxes.

Then there were the cowards: the 29 per cent who, when confronted with the question, "Who is going to pay?" now said that they did not know. Most of them were quite happy to have an opinion when they were just asked to complain about how expensive things are – when asked if the rise were reasonable or unreasonable, only 8 per cent said that they did not know.

However, I am not here simply to salute the bravery and honesty of the 5 per cent who declared, when asked the apparently cost-free question, that the fare rises were "reasonable". I am here to complain about the feebleness of our politics, which all too often seems to consist of lobby groups campaigning against how expensive things are, opposition politicians pointing at things and saying, "Gosh, that's expensive", and ministers pretending that they can make things cheaper with nobody having to pay the bill.

On Monday, for example, there was a fuss about the Government's plan to relax the rules requiring property developers to build a share of "affordable housing". Yesterday, the so-called Taxpayers' Alliance launched a campaign to point out that petrol is awfully expensive. And today the equally so-called Fair Tax on Flying campaign announces that it has persuaded 100,000 people to write to their MPs complaining that air passenger duty is "too high".

In none of these cases was the lobby concerned asked the equivalent of YouGov's second question, namely who they thought should pay to make their favoured commodity cheaper. The word "affordable" is one of the corruptions of contemporary language. What it means is that the price, usually of housing, should be lower than the market value, with the difference being made up by the taxpayer or, in this case, by the buyers of the other properties on the development. As for petrol, I think most people know that most of the pump price is tax, but there are good reasons for that and anyone demanding lower taxes on fuel really ought to say which other taxes should rise, what public spending should be cut or that the Treasury should borrow more. The same goes for air passenger duty. Of course people think it is "too high" or "unreasonable" when they have to pay it, but ask them how to make up the loss to public funds and you will get a different answer.

And that is before we even get to the second-order effects of price changes. Keep rail fares low and overcrowding on trains will get worse. Keep some house prices low and people will have more incentive to cheat the eligibility criteria, for first-time buyers or "key workers" or social housing. Cheaper petrol means more congestion and pollution. Cheaper flying means the same.

Thus our politics seems thin. At least the Labour Party resists the first temptation of opposition politics, which would be to concede to every lobby. That is particularly tempting at the moment, because so many respected economists say that it makes sense to cut public borrowing more slowly. However, if we do want to borrow more to keep the economy ticking over and to keep more people in work, we shouldn't do it by giving in to the loudest lobbies. Credit to Ed Balls for that, at least.

No credit to Ed Miliband, though, for his sound bite complaining about a "cost-of-living crisis". The Labour leader seems to suggest that we should be surprised or affronted that, in a recession, people find it harder to make ends meet, and that, somehow, the Government should pick some money from the money tree to make sure that no one (apart from bankers, as if there were no bankers under a Labour government) should ever be worse off.

It may be a good idea for the Government to borrow even more and give people vouchers to spend. They tried that in Japan, I seem to remember, and it did not succeed in getting the economy growing again. But it would be great to hear a politician say, just once in a while, that you cannot eat your cake and have it.

John Rentoul is chief political commentator for 'The Independent on Sunday'

j.rentoul@independent.co.uk