This article examines the economic nature of copyright law. The standard economic literature assumes that copyright represents a legal remedy to a market failure. Thanks to copyright authors have the ability to appropriate revenues from those who use their creations and in this light it represents an incentive to creation. On the other hand granting a copyright to an individual is not without societal costs. Ideas are public goods and their privatisation has several negative consequences: the exploitation of the exclusive power granted by copyright generates a monopoly which brings the usual dead-weight-loss and the usual exclusion of a consistent part of society from the consumption of ideas created. In addition there are a number of neglected side-effects which, if rightly inserted in the costs-benefits analysis, will lead to the conclusion that copyright, as it is today, can not solve the trade-off between private incentive and social welfare, while increases the inefficiency in the economic systems.