But doesn't this argument defeat itself? We cannot, a priori, know very much at all about the world. There is very, very little we can "know" with certainty -- that's the whole reason Descarte resorted to the whole cogito argument in the first place. You and GP just choose different lines to draw.
Yes, I agree completely. I think the apriori/aposteriori distinction is always worth making though.
This really does matter a lot more when floating signifiers get involved; I'm not actually contesting that our models of electrical engineering model reality quite well.