Full Idea
Our knowledge of concreta is a posteriori, but our knowledge of numbers, at least, has often been considered a priori.
Gist of Idea
We are thought to know concreta a posteriori, and many abstracta a priori
Source
Stephen Yablo (Abstract Objects: a Case Study [2002], 02)
Book Reference
-: 'Nous' [-], p.221
A Reaction
Personally I think numbers are rooted in experience, though pure arithmetic has travelled a long way since it started. I doubt whether arithmetic is possible without counting things. I don't think I believe in the 'pure' a priori.