an alleged conspiracy actually took place. The Supreme Court quite properly held that evidence of pricing behavior that is as consistent with an inference of competition as it is with an inference of conspiracy cannot survive summary judgment. Any other conclusion would enable competitors to use the antitrust laws to deter the very price competition the antitrust laws are supposed to encourage. This is not an arbitrary filter--just a sensible rule of proof. If a future plaintiff can produce real evidence of the type of predatory conspiracy Professor Williamson hypothesizes, Matsushita -- if properly interpreted -- should be no barrier.
In general, however, we are in agreement with the main point of Professor Williamson's essay that fluid legal process rather than fixed rules should govern antitrust. Economics is an enormously useful tool to help illuminate business behavior for antitrust policy makers, and the increasing use of economics by the courts is commendable. However, with only a few exceptions (such as the per se rule against horizontal price fixing), hard and fast antitrust rules, be they economic or legal, have generally outlived their usefulness.
If there is to be any new "presumption" in antitrust, therefore, it should be that business behavior will not be judged under immutable legal rules, but rather that existing rules will be illuminated by new economic learning and other important changes in the matrix of business decision making. Legal rules in antitrust should be presumptively subject to change when economic theory and other antitrust policy inputs so dictate. No high priest of current economic or legal thinking should imagine itself to have all the final answers by which antitrust problems will forever be judged.
The authors thank Shelley E. Harms for her assistance.