Full Idea
I argue not that our most basic rules are a priori or empirically indefeasible, but that we treat them as empirically defeasible and indeed a priori; we don't regard anything as evidence against them.
Gist of Idea
We treat basic rules as if they were indefeasible and a priori, with no interest in counter-evidence
Source
Hartry Field (Apriority as an Evaluative Notion [2000], 4)
Book Reference
'New Essays on the A Priori', ed/tr. Boghossian,P /Peacocke,C [OUP 2000], p.136
A Reaction
This is the fictionalist view of a priori knowledge (and of most other things, such as mathematics). I can't agree. Most people treat heaps of a posteriori truths (like the sun rising) as a priori. 'Mass involves energy' is indefeasible a posteriori.