Do we really want the courts determining when the fraction of the total value due to legal sales is high enough to exonerate manufacturers from contributory infringement? Neither the wisest courts nor wisest economists have enough knowledge to make that decision in a way that is likely to produce more benefits than harm. Does the fraction of legitimate value have to be higher than 50 per cent, 75 per cent, 10 per cent, or some other number? Courts should consider past trends in these percentages because new uses for say a software-legal or illegal- inevitably emerge over time as users become more familiar with its potential. Must courts have to speculate about future uses of software or other products, speculation likely to be dominated by dreams and hopes rather than firm knowledge? There is a possible middle way that should be considered, and that is to provide a safe harbor to potential contributory infringers who take all reasonable (cost-justified) measures to prevent the use of their product or service by infringers. The measures might be joint with the copyright owners. For example, copyright owners who wanted to be able to sue for contributory infringement might be required, as a condition of being permitted to sue, to place a nonremovable electronic tag on their CDs that a computer would read, identifying the CD or a file downloaded from it as containing copyrighted material. Software producers would be excused from liability for contributory infringement if they designed their software to prevent the