“For an especially simple example, note that Fort Knox has never been robbed. However, this does not mean the guards can safely be eliminated, since the incentive not to rob Fort Knox depends on the presence of the guards. In other words, with the heavy security that exists at the fort today, criminals are unlikely to attempt a robbery because they know they are unlikely to succeed. But a change in security policy, such as eliminating the guards for example, would lead criminals to reappraise the costs and benefits of robbing the fort. So just because there are no robberies under the current policy does not mean this should be expected to continue under all possible policies.”