[bwanab]

No economist believed that negative interest rates were formally (or informally for that matter) impossible. You might find arguments that real rates (nominal rate minus inflation) should never be negative, but in a deflationary environment nominal rates can go below zero without violating that.

[sergiosgc]

Yes, of course, in a deflationary environment, rates can be negative. We are in an inflationary environment, though. Zero was thought to be a hard limit -- in terms of interest rate efficacy as a lever to act on the economy.

Now, the mantra is that near zero negatives are still within the efficacy envelope. It was not predicted, but stated after the fact. This is, to me, a clear sign that the model is wrong, and should be reviewed with no barriers to questioning basic assumptions.