What, Exactly are we Trying to Tax?
Taxes serve the function of constraining consumption in order to offset the inflationary impact of government spending. You could boil that down to saying that taxes "pay for" spending, but that's an artificially limiting view. There are lots of ways to pay for it, and it doesn't all need to get paid for at once. Sometimes, it's much better to borrow—for example, during a recession deficits tend to expand due to unemployment insurance and often direct stimulus payments, bailouts, etc. Meanwhile, the government’s capacity to borrow goes way up, because investors buy safe assets like treasury bonds. So deficit spending is partly a way for governments to blunt the impact of a recession on the most economically vulnerable, but it’s equally fair to describe it as another instance of the cynical financial saying: "When the ducks are quacking, feed them."
In designing a tax system, it's tempting to think that what you're deciding is who to tax and how, and the usual intuition is that it's fair to tax people who have a lot of money, and not people who don't. But that hasn't always been the default policy; the oldest tax systems we have tended to be designed by the rich and powerful (in a system where land is the main source of wealth, and where wars are fought by untrained militias rather than professional soldiers, the money-power correlation is almost perfect). And those systems tended to tax frequently-consumed goods that just about everyone needed, like salt.
