Our decisions are based on our memories and statesmen who lead the nations use history as their guide. Without such knowledge a society would collapse. Other props, such as oracles, which guided the ancients, have been discredited. In their absence we rely either on analogies with past events or on extrapolation from recent trends, yet both are fallible. Booms turn to busts, population expansion to decline, military success to failure. Analogies are useful didactic tools and put current events into perspective but each case is unique and the issue is whether one event has sufficient in common with a past one to make analogy a useful guide. Historians are not immune from mistakes but those historians who say they should not participate in contemporary political debates leave the field open to the less well informed. Above all they need to tell those with power to be cautious because major policy decisions are at best a matter of well-informed guesswork.