In an interview with CNBC’s Squawk Box, billionaire investor Bill Ackerman said the Fed’s actions to curb inflation have not been effective and that inflation remains the economy’s biggest problem.

Though Ackman stressed that the economy is actually quite strong right now with full employment, wage increases, and plenty of jobs available, it’s hard to say whether that will be the case in 6 or 12 months. It’s also the lowest default rate in history, which has put well-capitalized people in a good position, but the lower-income population is definitely under pressure with higher gas and food prices.

The Fed needs to be aggressive and keep rates high, at least at 4% or even higher for the next 12 to 18 months, Ackman told CNBC. And the biggest risk to the market is that people are not factoring long-term, elevated rates in. The Fed needs “to take its foot off the accelerator,” because the current rate is still ratcheting up the economy and fueling the inflation that’s raging right now. Ackman also pointed out the irony that the trillions of dollars that were poured into the economy in order to help people most hurt by the pandemic is partly to blame for the record-high inflation that is hitting those same people the hardest.

Ackman is the founder and CEO of hedge fund management company Pershing Square Capital Management.