This thesis rejects the position, dominant in political philosophy since Plato that the authority of states may be explained by means of a moral theory of legitimacy. It denies that it is possible even in principle to determine a principle that can endow a state with the moral entitlement to rule and create for its citizens a moral obligation of obedience which thereby authorises it to coerce them. The thesis argues that a Lockean understanding of the state leads more naturally to the position that the state is properly understood as a necessary evil granted qualified justification to coerce in order to protect people from each other. It locates this ambiguity in the moral psychology of the individuals from which a Lockean state must derive its powers and through whom it acts. It further claims that, Government officials being no different in character than the individuals over whom they rule, further coercion may be justified to raise funds by taxation to set up political institutions such as a separation of powers, and to ensure that citizens may equip themselves with the skills needed to avoid being financially dependent on the state. This justification is nonetheless provisional, and the responsibility to weigh the necessity of public coercion against the evil that it involves falls upon individual voters as much as parliamentarians and prime ministers.