As the Fed enters the sixth year of its campaign to revitalize the economy, the debate between the Fed’s majority and Mr. Lacker — whose views are shared by others inside the central bank, as well as some outside observers — highlights the extent to which the Fed is operating in uncharted territory, making choices that have few precedents, unclear benefits and uncertain consequences.

The economy continues to muddle along, shadowed by the threat of another government breakdown, and the crisis of high unemployment is only slowly receding. But in trying to address those problems by suppressing interest rates, the Fed risks the unleashing of speculation and inflation.

It is basically a matter of disposition: is it better to risk doing too much, or not enough?

Mr. Lacker, 57, often uses the word “humility” in describing his views. He means that the Fed should recognize that its power to stimulate the economy is limited, both for technical reasons and because it should not encroach on the domain of elected officials by picking winners and losers.

As he sees it, the Fed’s current effort to reduce unemployment by purchasing mortgage-backed securities crossed both lines. He sees little evidence that it will help to create jobs. And he says that buying mortgage bonds is a form of fiscal policy, because it lowers interest rates for a particular kind of borrower.

But Mr. Lacker is at pains to emphasize that his disagreement with the other 11 members of the Federal Open Market Committee, who supported the purchases, is not about the need for help.