What is income, really? Ask an economist and they might describe “Haig-Simons” income—the value of a person’s consumption of goods and services, plus the change in their net worth over a certain period. A lawyer might refer to Section 61(a) of the IRS Code 26, which defines “gross” income as “all income from whatever source derived”, including but not limited to commission, interest, property deals and wages. An accountant might talk about how to reduce that gross income, via deductions or carve-outs, to a skinnier “taxable income base”.

The answer matters. Whether governments should levy taxes on unrealised capital gains, as well as realised ones, is a topic of hot debate. In March, during the State of the Union address, Joe Biden reiterated his commitment to imposing a “billionaire minimum income tax” if re-elected. This would include a 25% tax on unrealised capital gains for Americans with more than $100m in assets, which he expects would raise $500bn (2% of GDP) over a decade. The Supreme Court is also considering the question. Its justices are poised to issue an opinion in Moore v the United States, a case in which the plaintiffs are arguing that a one-off tax on gains from an overseas investment was unconstitutional, since the 16th amendment, which enshrines in America’s constitution the federal government’s right to impose income taxes, does not apply to unrealised income.