Full Idea
If we stipulate the meaning of 'the number of x's' so that it makes Hume's Principle true, we must accept Hume's Principle. But a precondition for this stipulation is that Hume's Principle be accepted a priori.
Clarification
Hume's Principle defines numbers by one-to-one correspondences
Gist of Idea
If we stipulate the meaning of 'number' to make Hume's Principle true, we first need Hume's Principle
Source
Paul Horwich (Stipulation, Meaning and Apriority [2000], §9)
Book Reference
'New Essays on the A Priori', ed/tr. Boghossian,P /Peacocke,C [OUP 2000], p.163
A Reaction
Yet another modern Quinean argument that all attempts at defining things are circular. I am beginning to think that the only a priori knowledge we have is of when a group of ideas is coherent. Calling it 'intuition' might be more accurate.