 Frege considered whether HP might be laid down as a constitutive definition of numbers—a definition that would give a full and exact characterization of their nature. But he concluded that HP could not meet this more exigent, yet reasonable, demand. The reason is what has now become known as the Julius Caesar Problem. Frege insisted (Grundlagen, §56), that our definition of number should enable us to decide that Julius Caesar is not a number. His conclusion was that HP could not enable us to do this. 