Seumas Milne kicks arse!
So, there's a moral argument for this that is hard to argue against. Put simply, everyone is equal and we all deserve an equal return on our investments- in terms of the effort put in. This is the essence of the free market- you earn what you work for. However, "money breeds more money", as the old axiom goes: If you have money already it requires less effort to acquire more than if you had to earn the same amount without any wealth to start with. Therefore, if you possess a large amount of money, you should be taxed more on any income because your earning potential is greater due to your initial wealth.
I think that's how it goes.
This article, which I found whilst randomly looking for an appropriate quote to support the axiom mentioned above, is edifying.
In the Green Party's Manifesto for a Sustainable Society I found this. I like.
"Direct Taxation - Income Tax
EC710 Income Tax is the instrument by which all citizens who are able to are required to contribute a proportion of their labours to the running of public services. It is also, when combined with benefits payments, the primary way in which wealth can be redistributed in order to create a fairer society.
EC711 Personal tax-free allowances will be abolished, having effectively been replaced by the Citizen's Income (see EC730). Income Tax will be levied on all income above the Citizen's Income. Tax rates will be banded and will increase progressively so that those on higher incomes are paying higher marginal rates of tax. In particular, rates higher than 40% will be introduced for those on the highest incomes.
EC712 In order that people are not penalised by paying high rates of tax in one year, whilst their income dramatically drops in the next (either through personal choice or for reasons beyond their control) income will be averaged over five years and the tax calculated on the rolling average figure.)"
Labour's approach to the subject is, predictably, somewhat less ambitious:
"Brown recently refused to rule out raising the top rate of tax. "We're still a very long way from that politically," one cabinet minister said yesterday. "There are powerful forces against us." For which read the bulk of the media and the most influential people in the country, who would all have to pay more tax."