Frege considered, but rejected, the idea that Hume's Principle, by itself, could be taken to define the concept of number. His rejection was based on what has come to be known as the “Caesar Problem”: Hume's Principle allows one to decide whether the number of Fs is identical with q, when q is given in the form ‘the number of Gs’, but it fails to do so when q is not given in that form. Thus, Frege (1884: §56) requires that we be able to 