In the theory of explanation it is imperative that self-explanation be excluded; we cannot allow an explanandum to explain itself. This applies to the explanation of events and to the explanation of laws. In Nagel's account the following three features of a valid explanation of a universal law serve as a prophylaxis against self-explanation: “... all the premises are universal statements, there is more than one premise, each of which is essential in the derivation, and the premises, taken singly or conjointly, do not follow logically from the explicandum”).