What did Donald Trump do today?

He forgot that he's not President of the U.S. Federal Reserve.

The Federal Reserve is widely expected to raise interest rates at its meeting this Wednesday. If so, it will be the fourth time it has done so this year. Raising rates has a series of related effects on the economy, but there are two that are typically the most important: it keeps inflation from rising too quickly, and it makes it possible to recover more quickly from future economic downturns. 

Trump, who needs better economic news than he's getting as a matter of political survival, erupted on Twitter at the news. It's not the first time Trump has insisted that the uncertain economic picture of the moment is really the Fed's fault—and not, say, the effects of the trade war he launched this year against most of the United States' major trade partners. 

Trump is entitled to his opinion, although his understanding of macroeconomics is famously limited. He once called his now-disgraced National Security Advisor, Mike Flynn, in the middle of the night to ask whether a strong or weak dollar was preferable. (As with interest rates, it depends on the situation.) Flynn suggested Trump call an actual economist.

But whatever Trump might wish for, the Fed is supposed to be independent from political demands, which are often in conflict with what is best for the economy. The only direct influence Trump is supposed to have over the country's central banking system is by appointing its chair and board of governors, which in turn makes the decision about raising or lowering rates. 

Trump has appointed four of the five currently serving members of the board of governors whose decisions have so upset him.

Why does this matter?

• The stability of the U.S. and world economy is more important than Donald Trump's political needs.

• Assuming you know more about a subject than anyone, all evidence to the contrary, is not a sign of good mental health (although it can be politically useful).