(JJ) If you are justified in believing that p, then p is warranted enough to justify you in phi-ing, for any phi. (Quoted from p. 99)
(Moderate Externalism about Justification) Justification does not supervene on the subject's internal states. In particular, external properties like reliability and Gettierizedness can make a difference in whether one is justified in a particular belief. (Paraphrased from p. 107)
Fantl and McGrath argue that (JJ) implies that 'purist fallibilism' about justification cannot be true. Now as I wrote a little while ago, I don't really buy into the notion of purism. And to be honest, I have some problems with the notion of fallibilism, too -- I'll try to write them up sometime soon. But set all that aside. The basic idea is that, if you accept (JJ), then you think that there might be two subjects that differ only in, for example, how important p is to each subject, such that one is justified in believing that p and the other is not. I guess I think that's right, although I'm thinking of things in a way different from the way Fantl and McGrath do.
Fantl and McGrath think that people who accept this and are also moderate externalists (hereafter 'externalists') "commit themselves to counterintuitive claims about action." First, Fantl and McGrath observe the familiar point that externalists think there could be intrinsic duplicates who differ in their justification facts; externalists, therefore they face the New Evil Demon problem. That's familiar stuff, and, as Fantl and McGrath say, there are many possible responses. But they think things get worse once you also accept (JJ). They write:
Moderate externalists who accept JJ not only have to say that two subjects who differ only in how reliable they are can differ in what they are justified in believing. They also have to say that the subjects can differ in what they are justified in doing. This is counterintuitive. (108)
It's not really clear to me that this is a counterintuitive verdict. But more to the point, I just don't see why Fantl and McGrath think externalists who accept JJ are thereby committed to it. They don't, as far as I can see, explain why they think this result should obtain. It plainly doesn't follow in any direct way from externalism and JJ; externalism says that external properties can influence belief-justification facts, and JJ gives one link between belief-justification and action-justification, but it's just nowhere near strong enough to imply, as Fantl and McGrath seem to think it implies, that external properties can shift action-justification facts.
Take subject LOW who is justified in believing p, and for whom p justifies Xing. Externalists are committed to the possibility, in at least some cases, of another subject, HIGH, intrinsically identical to LOW, who is not justified in believing p. Fantl and McGrath seem to think that externalists are committed by (JJ) to think it possible, consistent with these stipulations, that HIGH is not justified in Xing, but (JJ) just doesn't get them anywhere near that commitment. Indeed, (JJ) is silent about HIGH. This principle tells you about what happens when a subject is justified in believing p; it entails nothing about what happens when a subject is not justified in believing p. For all (JJ) says, p may justify HIGH in Xing, too. (Consider this coherent principle that entails (JJ): If anyone intrinsically identical to you is justified in believing p, then p is warranted enough to justify you in phi-ing, for any phi.)
Suppose we considered a stronger, biconditional, principle:
(JJ*) If and only if you are justified in believing that p, then p is warranted enough to justify you in phi-ing, for any phi.
I don't know whether (JJ*) is plausible or not; it's strictly stronger than the principle Fantl and McGrath defend. It gets around the problem I just raised for their charge against the externalist. But even (JJ*) isn't strong enough to deliver an entailment from externalism to a difference in what actions are justified between LOW and HIGH. Externalism and (JJ*) commit one to the verdict that p cannot justify HIGH in Xing, even though it can justify LOW in Xing. That's a far cry from the stated claim that nothing justifies HIGH in Xing. And I just don't see any plausible argument that this could be the case. It may be, for all (JJ*) says, that HIGH and LOW must be justified in performing all the same actions, but that they have divergent propositions justifying those same actions. (The plausible way to develop this line, I think, is that HIGH's reasons are a proper subset of LOW's.)
So I don't think that externalists who like (JJ), or even those who accept (JJ*), are committed to the allegedly counterintuitive claims about action that Fantl and McGrath charge.