- (Neg) S's belief that p is justified a priori if and only if S's justification for the belief that p does not depend on experience.
- (Pos) S's belief that p is justified a priori if and only if S's belief that p is justified by some nonexperiential source.
Thus stated, the negative characterization (N), Al says, is ambiguous, because of different ways in which justification can depend on experience. So (N) is disambiguated into:
- (Neg-Weak) S's belief that p is justified a priori if and only if S's belief that p is nonexperientially justified.
- (Neg-Strong) S's belief that p is justified a priori if and only if S's belief that p is nonexperientially justified and cannot be defeated by experience.
Of the three remaining views -- Pos, Neg-Weak, and Neg-Strong, Al goes on to claim that there are really only two, because Pos and Neg-Weak are equivalent. The rationale here seems to be the idea that every justified belief has its justification due to a source, and any given source is either experiential or nonexperiential. Take a justified belief, and consider the binary question of whether its justification's source is experiential; Neg-Weak says yes if it is; Pos says no if it isn't.
But I think this bit of reasoning is too quick. I'm suspicious of the move from nonexperiential justification to derivation from a nonexperiential source. To equate Pos with Neg-Weak is to legislate in advance that for any justified beliefs, there is a source of its justification. That is to say, it assumes prior to argument that there is no original justification -- justification that does not depend on a source. But that there is such original justification is, it seems to me, a coherent view that occupies a spot in logical space. (For what it's worth, I also think it's true; Ben and I defend it in The Rules of Thought.)
Sources generate things that weren't already there. The assumption that justification for a priori justified belief must derive from a source is, I think, part of the motivation for supposing there must be some kind of faculty of intuition to serve as source.
I'm not sure whether there are nonexperiential sources of justification. But I'm firmly committed to beliefs that are justified in a way that doesn't depend on experience. If these two attitudes are jointly coherent, then Casullo is wrong to equate Pos with Neg-Weak.