Count it, don't follow it
Economists have one simple idea to contribute to the MP expenses saga. You never know, it might change the way you think about the whole thing.
The idea is this: money is fungible. Put simply, that means one £50 note is the same as any other. They all have the same value and they will all buy the same amount of stuff.
Put it even more simply, if you give someone £100 to buy a chair, you can't say for sure that he bought a chair with your money - even if he shows you the chair, and a receipt. All you can say for sure is that you made him £100 better off, and he has bought a new chair.
This might sound completely obvious. Or completely stupid. Maybe both. It depends on how much time you spend with economists (or lawyers). But this idea of fungibility gives someone with an economist's frame of mind a slightly different perspective on the revelations of the past week.
Voters are understandably angry at the thought that MPs might have been "playing the system" - and even more angry when they appear to have claimed taxpayers' money on false pretences.
But the focus, at least until recently, has mainly been on who claimed for what, and whether it was an "appropriate use of taxpayers' money" - the moats, the dog food, the fancy furniture.
You can see why those claims would catch the eye. I'm pretty fascinated by them too. But the less prurient, economist, side of my brain is less interested in the details of the MPs' claims than in how much they got.
If, like so many MPs, your MP has claimed the full amount - around £24,000 in the most recent year, then the point to note is that he or she has had £24,000 a year more to spend. Full stop.
The stories tend to focus on what the MPs say the money was spent on - and the receipts. But you might just as well ask what they did with the chunk of their salary that was no longer taken up with everyday bills - or how much of that money they would have spent, even if they weren't MPs.
Of course, all these questions overlap, and people have been interested all of them, even if the iffy expense details have grabbed the headlines.
For example, some have said it was unfair that MPs such as David Cameron have escaped criticism for claiming the maximum allowance each year, because their claims were almost entirely made up of mortgage interest and utility bills. Whereas other MPs, possibly with similar necessary expenses, have made much smaller claims, yet faced criticism for the details.
David Cameron could be spending his allowance on underground swimming pools and platinum cycle helmets. All we know is that he has utility bills and mortgage interest to pay of more than £20,000 a year.
Experts in overseas aid know the problem of old. When the World Bank or an official development agency gives money to a developing country it usually says it wants the money to go to particular priority areas - like women's education or primary healthcare. But, as they've learned to their cost, the recipient government often has other ideas.
"When you give $1bn to a developing country", a World Bank economist once said to me, "you may think you're giving $1bn to your pet project, but the reality is you're giving it to the president's."
Because money is fungible, the $1bn might appear to go into the education budget, but it frees up $1bn of domestic revenue that could just as easily turn into a new presidential jet.
On the basis of the past week, you might say the same about the expense allowance of some MPs.
It's a very old problem in development aid - and, over time, the likes of the World Bank have come up with ever more complex arrangements to solve it. They'll probably come up with an elaborate solution for MPs as well.
But you still have the basic problem that money is fungible. You never really know what people would have spent if the allowance didn't exist - or if they weren't, in fact, MPs. Though in the case of some of them, we may be about to find out.