I know it when I see it.
Justice Potter Stewart of the US Supreme Court gave one of history's least fulfilling answers when he was asked to define "hard-core pornography"; but the grain of truth contained within is important.
The same temptation to throw one's hands up at the difficulty of defining complex phenomena is everywhere. The Economist's Daniel Knowles, for instance, suggests that it applies to poverty  while the Sorites paradox  – a close relative – attaches the problem to bald men and heaps of wheat.
It also applies, pretty much perfectly, to tax avoidance.
We all know it when we see it. Take, for example, Polly Toynbee's column from the Guardian today :
The big sell is trusts, special ones devised for this company's clients, guaranteed to protect almost all your wealth from inheritance tax. They are right, it can be done easily. Put all moveables and all cash and investments into a discretionary trust, and it passes to your heirs without tax as soon as you die, not even waiting for probate. It counts as a gift so the beneficiaries need pay no tax either. Called a "discretionary trust", as technically St James's are the legal trustees, the discretion in fact remains in all but name with you: the company will do whatever you ask, so you still control the fund and you can still take money from it. But for reasons that defy basic tax fairness, it avoids all inheritance tax. Why?
Since New York City tax laws don't apply to people who are deemed to be nonresidents, even if they own a residence in the city and work there, Robertson was allowed to spend no more than half a year – a hundred and eighty-three days – in New York City. This exile was self-imposed. If he had paid New York City tax, which in the top bracket reaches a rate of 3.6 per cent of taxable income, he could have spent as much time in the city as he wished...
Friday nights were particularly risky, since Robertson or his wife often had social events scheduled in the city. In order to "earn a tax day," as he put it, he usually left town on Friday before midnight, even if his wife stayed at the apartment. Robertson's driver had to be on alert: as long as they crossed the Queens border en route to Locust Valley by midnight, Robertson didn't have to "waste" a Saturday as a New York day. Even one minute of a day spent in the city counts as a day of residence. (Exceptions are made for people who are in transit from one destination outside the city to another – from Newark airport to Long Island, for example, or to LaGuardia for a flight.) Robertson said he never missed the midnight deadline, although when he couldn't get his driver or a limousine service in time he occasionally had to hail a cab. On one occasion, Robertson came back from a trip and found himself crossing into Manhattan at 11:45 P.M. That mistake cost him a full New York City day, which he could have avoided by whiling away fifteen minutes at the airport.
Or the three multinationals hauled up in front of the Public Accounts Committee , about whom Richard Murphy writes:
For Amazon things were much worse. Their rep could not justify how an order made in the UK for a product in a UK warehouse, shipped by UK staff through the UK post and with a bill enclosed printed in this country could somehow have anything to do with Luxembourg when so very obviously it hasn’t. Despite this he had the gall to claim tax must be paid where the economic substance of the deal is – even though Amazon does nothing of the sort…
Google tried harder but they had created one insurmountable obstacle for themselves. Their argument was profits should be taxed where they are earned and they said US technology drove their European profits. But for their admission that the payments made from Europe for that technology never reach the USA and instead get parked in tax-free Bermuda ended whatever shred of credibility they’d tried to create.
All of these things are as clearly tax avoidance as Reader's Wives is clearly pornography. The problem comes when you try to come up with a definition which encompasses all of these examples while not also covering whatever the taxation equivalent of Last Tango in Paris is.
You can try to define it as acting to deliberately minimise your tax take – but then, what is taking out an ISA? That is an action which is performed for no other reason than the tax benefits, but it's clearly not tax avoidance.
There must, then, be some definition of the spirit of the law. Loopholes in tax are put there for a reason, but sometimes that reason is tricky to specify completely. So, for example, the loophole that investment income in taxed less that earned income exists to encourage people to invest their money (which is good for growth) – but when hedge fund managers are payed through "carried interest", that gets classed as tax avoidance, because it is technically investment income, but hasn't actually required any investment from the people benefiting.
Unfortunately, that definition doesn't work either. The absence of VAT on books, for example, is to promote an educated, well-read population; but even though 1001 reasons Britain is shit doesn't do that, we don't call it tax avoidance.
The problem persists even if you just look at specific examples of avoidance. Multinational corporations, for instance, sometimes headquarter themselves in of tax havens. Other times, they leave their headquarters where they are, but manipulate their accounts so that it looks like all their profits come from tax havens. Tempting as it is, it's very tricky to come up with a catch-all definition of avoiding behaviour in this situation.
Is it "not paying tax where you are headquartered"? Or is it "not paying tax where the money is earned"? Or is it a third, "pretending money is earned in one place, when it's really earned in another one"? Or a fourth, "paying tax in a tax haven"? Or even just "operating out of a tax haven"?
Perhaps the real solution is to just stop trying. Call out egregious examples of tax avoidance, but resist the lure to dictate a full definition of the term. Make clear to those who set policy that building a tax code which is easily abused will result in protest, and that avoiding tax will result in bad press. But save definitions for the courts, because it's a fight which seems nearly impossible.