 The definition of freedom as a triadic relation was first put forward in the seminal work of Felix Oppenheim in the 1950s and 60s. Oppenheim saw that an important meaning of ‘freedom’ in the context of political and social philosophy was as a relation between two agents and a particular (impeded or unimpeded) action. However, Oppenheim’s interpretation of freedom was an example of what Berlin would call a negative concept. What MacCallum did was to generalize this triadic structure so that it would cover all possible claims about freedom, whether of the negative or the positive variety. In MacCallum’s framework, unlike in Oppenheim’s, the interpretation of each of the three variables is left open. In other words, MacCallum’s position is a meta-theoretical one: his is a theory about the differences between theorists of freedom.