Causal conditions in and of themselves can't give us justifications for a belief. They can, however, determine the nature of belief. As far as justification is concerned, we need to make the causal conditions justify our beliefs. That is, we need to say why or how such particular causal conditions have contributed to our true or false beliefs.
As Donald Davidson said, "causation is not itself under an aspect". It doesn't explain or justify anything. It only does so in conjunction with the epistemic practices of epistemologists who make sense of causal conditions and who also offer us the reasons why particular causal conditions - rather than others - determine the truth of our beliefs rather than their falsehood.
Reliabilists and externalists say that causal mechanisms in the brain hook up with stimuli that produce belief. Though do they hook up with stimuli that cause true belief? What makes the stimulations, casual mechanisms or whatever else cause true belief? Indeed do we even know how they cause any kind of belief - true or false?
The brain scientist can see what’s going on in the brain; though he can’t see the relation between the brain and the mind that makes sense of what goes on in the brain and what the brain gives it. Causes are precisely that - causes. And reasons are reasons. Reasons are in the domain of mind and causes are in the domain of the brain and world. Of course reasons themselves may be dependent on causes; though this wouldn't mean that reasons are nothing over and above causes.
The brain scientist can never see why we believe that Tony Blair is a politician. He can't even see the belief that P. Tony Blair being a politician or not lying is outside the brain. However, the mind - though not the brain - can both register Tony Blair being a politician or not being a politician and make sense of whether he is or is not a politician.
The brain scientist can tell us what subserves such beliefs (or even what causal mechanisms lead to this belief). However, he can't tell us why or how we come to believe that Tony Blair is a politician. Indeed we would need to tell him what the belief is and where it came from in order for him to tell us which causal mechanisms and brain states subserve such beliefs and reasoning processes. Without such prior knowledge, the brain scientist would simply be mucking about in the brain not quite knowing what it is he's trying to find or explain.
Of course causal conditions in and of themselves can't tell us everything we need to know about knowledge and knowledge-acquisition. How can they? Such conditions need to be explained and interpreted. Not only that: they need to be questioned and criticised. These causal conditions don’t just enforce themselves on the mind of the investigator. And they certainly shouldn’t enforce themselves on the minds of epistemologists. If they did, then there would be no such thing as epistemology. And there would be no such thing as empirical investigation either.
To give a simple example.
Which causal conditions are we talking about? There is an indefinite number. And if we choose certain causal conditions, the epistemologist can then ask:
i) Why have you chosen to concentrate on these and not the many others?
ii) Why is this causal condition relevant and the one you ignored irrelevant?
iii) How do you know that these causal conditions give us knowledge and the many others don’t?
iv) How do you know that you're looking at them in the right way?
Epistemology is different to science or empirical investigation. However, does even the most naturalistic epistemologist think otherwise? The naturalist simply says that we must rely on - or even defer to - empirical investigation or simply use such findings. How could it be any other way? If this weren’t the case, then epistemology would not really have a subject matter to clarify and elaborate upon. The pure Cartesian epistemologist may survive without science. Though what sort of survival would it be? It's a kind of survival that's individualistic or subjectivist. That it, the Cartesian epistemologist relies on himself and himself alone, at least in principle. So what’s to stop him making massive mistakes and barking on many false trees?
It's the Wittgenstein argument: if there’s no one available to tell him that he has gone wrong, then how does he no that he has gone right? The world is bigger than his own mind, no matter how great and systematic his mind is.