A priori arguments and materialism

This begins a series of responses to Keith Parsons on the Argument from Reason, found here. 

Parsons' first critical response to the argument is this: 

First, note that Goetz and Taliafero’s arguments, and practically all arguments against the MTB thesis, are a priori in nature, whereas the arguments for MTB are mostly empirical. Historically, a priori arguments have fared very poorly when opposed to empirical arguments. Philosophers will draw an a priori line in the sand and scientists will gleefully jump over it. The dismal track record of a priori claims against empirical ones provides some reason to doubt the cogency of arguments like those of Goetz and Taliafero.

Are all a priori arguments bad? Really? What about Godel's Incompleteness Theorem, for crying out loud? Or the entire edifice of mathematics, on which the entire operation of science is based? That's ALL a priori. 

It might be helpful to see some examples of what Parsons is talking about.