There is, understandably, a backlash against the government's proposal that women who want to claim tax credits for a third child must prove they have been raped. For me, this highlights the importance of the John Cushnie principle.

Mr Cushnie was a star of Gardeners' Question Time whose reply to many questions was: "it's not worth the bother."

This answer applies to a lot of the welfare state. Maybe it is a good idea to limit tax credits to two children. But is it worth the bother, in terms of bureaucracy and intrusion and bad PR? Maybe you think claimants should prove they are seeking work. But is it worth the huge cost of finding out? Maybe benefits should be sensitive to variations in need? But again, is it worth the administrative cost?

Perhaps your ideal welfare state would entail many complicated gradations of need and desert. But these run against the Cushnie priniciple: it's not worth the bother. For me, one virtue of a citizens' income is that it avoids the deadweight cost of administering so many fine distinctions.

Of course, the Cushnie principle broadens. It's one reason why I advise my readers to hold tracker funds. Granted, it's possible that momentum, value, defensive and quality stocks would, over time, out-perform. But it might not be worth the bother of complicating one's investments so much.

Readers will recognise the Cushnie principle. It's an expression of Herbert Simon's theory of satisficing. We often lack the knowledge and rationality to maximize, he said, so why not make do with reasonable simple rules? But the Cushnie principle goes a little further, by seeing that an attempt at maximization might not just be impossible, but counter-productive. For example:

- There's the unnecessary labour in gardening or investing, policy-makers equivalent of which is the cost of bureaucracy. Of course, true maximization would take account of these costs. But it's easy to under-estimate them - thanks to the planning fallacy - as Iain Duncan Smith discovered with universal credit. How often do politicians say: "the cost of this scheme has come in way under budget"?

- In politics, the pursuit of the best policy might lose one political capital, which would be better expended upon higher priorities.

- Trying to maximize can make us vulnerable to cognitive errors. One virtue of tracker funds is that they protect us from the countless biases that worsen investment performance.

- The best can be the enemy of the good. In that wonderful film Whiplash Terence Fletcher's pursuit of genius destroys the lives of musicians who would have had perfectly decent careers. Similarly, investors who chase high returns end up over-paying for poor performing lottery-type stocks. And as John Kay has shown, attempts to maximize shareholder value can lead to corporate failure, for example by demotivating employees and encouraging corruption.

In these ways, the Cushnie principle has many applications. There's more wisdom in gardening than in politics.