Sometimes arguments are made for voluntary funding of defense by way of attacking taxation. Anarcho-capitalists often argue, for instance, that the argument that taxation is needed to fund protection of liberty and property from aggression is a contradiction, because they believe taxation itself "requires aggression in order to be enforced". Another argument used by anarcho-capitalists states that, unlike voluntary transactions, no demonstrated preference has been made by the taxpayer; so they believe there is no objective way of showing that they are receiving a service they want and need, at a price that is fair.