So, yes, we have been here before. But it is a big assumption, and quite possibly a wrong one, to think that this is 1999 all over again. Quite conceivably it is actually 1995, when Alan Greenspan, then chairman of the Federal Reserve, was warning us all about “irrational exuberance”. Maybe he was right, but he was certainly too early. Markets stayed exuberant for another four years.

This week I read a prediction that the S&P 500 would rise over the next five years from today’s 4,700 to 8,000. That sounds preposterous, but it is not a finger in the air guess. It is a possibility based on historical patterns. The US benchmark typically swings from a level well below its long-term trend line to a cyclical peak that is well above it.

In the late 1940s, after the Depression and the Second World War, the S&P 500 stood 46pc below trend and 20 years later it was 88pc above it.

In 1982, after the devastation of the 1970s stagflation, it was 53pc below that same trend line and in 2000 it peaked 104pc above it. In March 2009, after the financial crisis, the index was 47pc in arrears; today, it is just 42pc ahead. That’s where the 8,000 target comes from. It may sound ridiculous, but it would not be unusual.

There are other reasons to think that today’s FOMO is not the red flag that a growing number of investors believe.

First, the exuberance remains relatively contained and focused on a few frothy areas of the market. While there were some “old economy” stocks that failed to participate in the 1999 boom, they were the exception that proved the rule. By the end, pretty much anything was in bubble territory. That’s not the case now.

Second, policy remains accommodative. Excessively so, some observers like former Pimco boss Bill Gross are now suggesting. Third, valuations, again with some notable exceptions, are not eye-wateringly high.