Prior to 1960, Hayek hardly makes reference to the term ‘rule’. In fact he manages to discuss cognitive psychology in The Sensory Order (1952) without recourse to this term at all. In The Constitution of Liberty (1960), he begins to use it, but without any suffix. In 1962 he adds a suffix and uses the phrase ‘rules of action’. By 1967 he replaces this phrase with ‘rules of conduct’, and continues to use it in all his later works. I suggest that there is more to this than just semantics.