In an interview with CNBC’s Squawk Box, professor of finance at the University of Pennsylvania’s Wharton School of Business Jeremy Siegel said that we are already in a mild recession, and urged the Fed to exercise caution in its tightening policy.

There’s been negative GDP growth in the first half of this year, and while a recession is defined has having two consecutive quarters of negative growth, we are certainly in a slowdown, Siegel stressed. That slowdown will continue into the 3^rd quarter, he said, and while inflation is gradually coming under control, “official inflation” will still be in the statistics over the next 6-9 months, especially given the disparities with how the Bureau of Labor Statistics prints out the inflation in different sectors. BLS inflation print outs are actually weeks or even months behind actual inflation, and the Fed must recognize that it’s going to get “bad prints” that represent past inflation numbers that need to work their way through the system.

While “we have to go through with [interest rate] increases,” Siegel told CNBC, he urged the Fed to slow down the hikes if the economy warrants it. The central bank made a big mistake in tightening so quickly and aggressively after waiting too long, he maintained, and it can’t be reversed without completely pummeling the economy. “…history of policy is a pendulum that often swings too far in one direction—too much ease,” he said, “and then panics and swings to too much tightening,” ending with the advice to “keep alert to those facts.”