This thesis argues that within political philosophy, property rights deserve closer attention than has been paid to them recently because the legitimacy of a state rests upon their definition and enforcement. In this way property rights differ from the right to liberty or equality. A state may or may not have liberty or equality, but it has no meaning at all if it does not enforce the rights of property. This is not to suggest that normative arguments for property rights are ‘nonsense upon stilts’. Morality may provide many reasons for an individual to exclude other members of a political community from a property. However, the function of property rights is to enforce that exclusion and this suggests that the normative legitimacy of a state is closely bound both to its ability to enforce whatever property rights it already has granted, and its justification of decisions taken when property rights are granted within its borders. My argument is that a proper political philosophy of property rights should acknowledge that a state depends upon its treatment of property rights for justification, not as a matter of justice, but as a matter of its existence.