When Nicola Sturgeon stood in the shadow of an indoor climbing wall this week, one immovable object in front of another, and launched the SNP manifesto, it didn't take her long to use some of her favourite phrases.

Nationalist MPs would build an alternative to austerity, she said, and deliver "progressive change" for the whole of the UK, by which she means, of course, more public spending and more taxation. But why does "progressive" have to mean what the SNP says it means? There is an alternative to the alternative: progressive change based not on increasing taxation, but on cutting it.

Until very recently, it was an argument the SNP itself believed in. The White Paper for independence included a promise to cut corporation tax and suggested such a cut would create jobs and attract firms to Scotland. For a long time, the Finance Secretary John Swinney was also very evasive about any commitment to raise the higher rate of income tax to 50p. It was only when Sturgeon began her pre-election dance with Ed Miliband that the SNP finally switched to Labour's policy and pledged its support for raising the top rate.

In its current mood, and with its current leader, supporting the 50p rate was a rather predictable move for the SNP to make even though the case for cutting tax is much more convincing, morally, economically, and socially. As David Cameron said recently, the moral case for low tax is based on a simple principle: we should keep as much as possible of what we earn so we can spend it how we like rather than have governments spend it on our behalf.

The other moral problem with tax is that it is, however you dress it up, a form of coercion (no one pays tax willingly) that raises money to be spent on things taxpayers are sometimes ideologically opposed to. Thomas Jefferson put it this way: to compel a man to pay for the propagation of opinions which he disbelieves or abhors is tyrannical.

And the moral case for low tax does not end there. In fact, there is a strong case, founded on fairness, against the 50p rate of income tax that Nicola Sturgeon is suddenly so keen on. Shouldn't we be allowed to keep the majority of our income? Anything else is unfair, isn't it? In fact, I would go further and argue that any tax over 25 per cent, no matter how rich you are, is morally dubious.

The economic case for low tax is just as convincing. Just a few weeks ago, the Scottish Government was jumping up and down demanding tax cuts for oil and gas firms and they were talking good sense. In the end, the chancellor George Osborne duly made the tax cut, from 30 to 20 per cent, and it is very likely to stimulate production and development in an industry that desperately needs both.

But can we really talk about tax cuts when the deficit is so high? If it is irresponsible of the SNP to talk about increasing public spending when we need to pay off a deficit - and it is - then surely it is even more irresponsible to talk about cutting tax? Actually, no. You might think cutting tax while trying to deal with the deficit sounds like a contradiction, but why should it be? Cut tax and people have more cash in their pockets and so they need to rely on government services much less. That in turn means government spending can be cut. In other words, cutting tax could help cut the deficit.

The third and final argument for low tax - the social and progressive one - is in many ways the most convincing. The assumption has always been that the only way to change society for the better is by raising taxes, and having governments spend the money for us. The SNP manifesto is based on this assumption. But it ignores the fact that cutting tax could be even more effective in changing society for the better, if only we tried it.

I can think of quite a few ways in which the progressive power of tax cuts could be used to encourage people to behave in a better way. Take schools for example: thousand of middle class parents move house to get their children into so-called better schools, but why not offer parents a tax break to send their children to the local comprehensive? We could go even further and offer people tax cuts to move from well-off postcodes to poorer ones, which would encourage better, more mixed, wealthier communities. There are lots of ways progressive tax cuts could work.

It would only take one politician to start talking in these terms for the idea to take off, but it will require some courage because we have to tackle the traditional assumption, celebrated and promoted in the SNP manifesto, that the only way to do good is by taxing people more. Change the way we tax people instead, and you can change the way people behave and make society a little better too.