In praise of unstable fixed points: the way things actually work

In recent years a fashion has grown up to ascribe great importance to "quantum critical points" at T = 0, at the boundary between the basins of attraction to the stable fixed points of ordered ground states. I argue that more physical significance in connecting microscopic interactions with observed phenomena lies in the common phenomenon of partially ordered "liquid" states at higher temperatures, unstable phases which define the relevant degrees of freedom and may order in many different ways as the temperature is further lowered. (C) 2002 Published by Elsevier Science B.V.