WASHINGTON - Federal Reserve Chairman Alan Greenspan said Friday that monetary policymakers face the inescapable fact that they must make decisions in a world of uncertainty.

Greenspan called this "inescapable uncertainty" the defining feature in the conduct of monetary policy, saying central bank officials are always being forced to make decisions based on an ability to know what twists and turns may occur in a complex economy.

"While change itself is certain, what form it will take next is always uncertain," he said.

Greenspan spoke at the opening of a two-day Fed conference designed to explore monetary policy.

He said nothing in his brief remarks abut the current state of the U.S. economy. The Federal Reserve at its meeting last month left interest rates at a 45-year low amid widespread expectations that the central bank will keep rates low for a considerable period of time in order to give the economy a chance to gain momentum.

In the latest economic reports, the government reported Friday that retail sales were down for the second straight month in October after a consumer shopping spree over the summer and that prices at the wholesale level rose by 0.8 percent, the largest increase in seven months.

In his opening comments, Greenspan said that there were many sources of risk and uncertainty in the economy and that it was important for monetary officials to have the input of economic researchers from around the world who are striving to understand better how the global economy operates.

"Economic understanding will always move forward but as we move forward, we realize how much we still do not understand," he said.