Brad Delong is making sense: "What I do know is that the way the issue is usually posed is wrong. People claim that Greenspan's Fed "aggressively pushed interest rates below a natural level." But what is the natural level? In the 1920's, Swedish economist Knut Wicksell defined it as the interest rate at which, economy-wide, desired investment equals desired savings, implying no upward pressure on consumer prices, resource prices, or wages as aggregate demand outruns supply, and no downward pressure on these prices as supply exceeds demand.
On Wicksell's definition -- the best, and, in fact, the only definition I know of -- the market interest rate was, if anything, above the natural interest rate in the early 2000's: the threat was deflation, not accelerating inflation. The natural interest rate was low because, as the Fed's current chairman Ben Bernanke explained at the time, the world had a global savings glut (or, rather, a global investment deficiency).