‘[S]o act externally that the free use of your choice can coexist with the freedom of everyone in accordance with a universal law’ (MM 6: 231). This is Immanuel Kant's first principle of justice, stated in the imperative form appropriate for human beings, beings who can comply with it but who might not do so. For Kant it is a principle that applies not only to relations among citizens within a state, but to those among states themselves and among citizens of varying nationality. As Kant's Rechtslehre makes clear, the universal law of justice, as he terms it, lies at the foundation of a set of standards that together form his theory of justice. Subsidiary standards follow from this most fundamental one by argument and together form a system, or metaphysics, of related principles. The system is hierarchical, that is, we can argue from the universal law of justice to increasingly concrete standards that help us apply it to various questions and in varying contexts.