This is alarmingly ill-thought out:

Oxfam’s version is particularly ill-thought out: “a one-off 99pc levy on billionaires’ wealth gains during the pandemic”.

As is normal - as a result of the creation of a political statistic - these days the measurement of gains is done from March 18 2020. Which was the lowest point for the S&P 500 following a 29% fall as the arrival of the pandemic became obvious. That’s being more than usually aggressive in loading the number by careful choice.

But think a little larger. Taxing - as we do with capital gains taxes - when people crystallize a gain seems reasonable enough. At which point we also allow, as we must, people to count their losses as well. The demand here is to tax uncrystallized gains. At which point we’ve also got to untax uncrystallized losses.

Which produces an interesting example. Bill Hwang just lost $20 billion as Archegos imploded. What’s his tax refund under this new idea then?

That is, it’s entirely true that some billionaires made money over the past 15 months or so. Some other billionaires lost money over that same past 15 months. Just the basic idea of the rule of law insists that profits and losses on the same issue get treated equally.

It’s not actually obvious that taxing changes in wealth over the course of the pandemic would raise any money at all. Which is a pretty bad advertisement for a new tax really, isn’t it?