just as books and bras are produced today. There could be competition among different brands of law, just as there is competition among different brands of cars". Friedman says whether this would lead to a libertarian society "remains to be proven". He says it is a possibility that very un-libertarian laws may result, such as laws against drugs, but he thinks this would be rare. He reasons that "if the value of a law to its supporters is less than its cost to its victims, that law ... will not survive in an anarcho-capitalist society". Anarcho-capitalists only accept the collective defense of individual liberty (i.e. courts, military, or police forces) insofar as such groups are formed and paid for on an explicitly voluntary basis. However, their complaint is not just that the state's defensive services are funded by taxation, but that the state assumes it is the only legitimate practitioner of physical force—that is, they believe it forcibly prevents the private sector from providing comprehensive security, such as a police, judicial and prison systems to protect individuals from aggressors. Anarcho-capitalists believe that there is nothing morally superior about the state which would grant it, but not private individuals, a right to use physical force to restrain aggressors. If competition in security provision were allowed to exist, prices would also be lower and services would be better according to anarcho-capitalists. According to Molinari: "Under a regime of liberty, the natural organization of the security industry would not be different from that of other industries". Proponents believe that private systems of justice and defense already exist, naturally forming where the market is allowed to "compensate for the failure of the state", namely private arbitration, security guards, neighborhood watch groups and so on. These private courts and police are sometimes referred to generically as private defense agencies (PDAs). The defense of those unable to pay for such protection might be financed by charitable organizations relying on voluntary donation rather than by state institutions relying on taxation, or by cooperative self-help by groups of individuals. Edward Stringham argues that private adjudication of disputes could enable the market to internalize externalities and provide services that customers desire. In the context of revolution, Rothbard stated that the American Revolutionary War was the only war involving the United States that could be justified. Some anarcho-capitalists such as Rothbard feel that violent revolution is counter-productive and prefer voluntary