Full Idea
We can never decide by means of our definitions whether any concept has the number JULIUS CAESAR belonging to it, or whether that same familiar conqueror of Gaul is a number or not.
Gist of Idea
Our definition will not tell us whether or not Julius Caesar is a number
Source
Gottlob Frege (Grundlagen der Arithmetik (Foundations) [1884], §56)
Book Reference
Frege,Gottlob: 'The Foundations of Arithmetic (Austin)', ed/tr. Austin,J.L. [Blackwell 1980], p.68
A Reaction
This has become a famous modern problem. The point is that the definition of a number must explain why this is a number, and not something else. Must you mention that you could use it to count? Count you count using emperors?