Suppose a rational agent _S_ has some evidence _E_ that bears on _p_, and makes a judgment _J_ about how _E_ bears on _p_. The agent is aware of this judgment, so she could in principle use its existence in her reasoning. Here’s an informal version of the question I’ll discuss in this paper: How many pieces of evidence does the agent have that bear on p? Three options present themselves.

1. Two – Both _J_ and _E_.
2. One – _E_ subsumes whatever evidential force _J_ has.
3. One – _J_ subsumes whatever evidential force _E_ has.

This post is about option 3. I’ll call this option JSE, short for Judgments Screen Evidence. I’m first going to say what I mean by screening here, and then say why JSE is interesting. Ultimately I want to defend three claims about JSE.

1. JSE is sufficient to derive a number of claims that are distinctive of internalist epistemology of recent years (meaning approximately 2004 to the present day).
2. JSE is necessary to motivate at least some of these claims.
3. JSE is false.

This post will largely be about saying what JSE is, then some arguments for 1 and 2. I’ll leave 3 for a later post!

Screening

The idea of screening I’m using here comes from Reichenbach’s _The Direction of Time_, and in particular from his work on deriving a principle that lets us infer events have a common cause. The notion was originally introduced in probabilistic terms. We say that _C_ screens off the positive correlation between _B_ and A if the following two conditions are met.

1. A and _B_ are positively correlated probabilistically, i.e. Pr(A | _B_) > Pr(A).
2. Given _C_, A and _B_ are probabilistically independent, i.e. Pr(A | _B ∧ C_) = Pr(A | _C_).

I’m interested in an evidential version of screening. If we have a probabilistic analysis of evidential support, the version of screening I’m going to offer here is identical to the Reichenbachian version just provided. But I want to stay neutral on whether we should think of evidence probabilistically. In general I’m somewhat sceptical of probabilistic treatments of evidence for reasons Jim Pryor goes through in his Uncertainty and Undermining (PDF). I mention some of these in my The Bayesian and the Dogmatist (PDF). But I won’t lean on those points in this note.

When I say that _C_ screens off the evidential support that _B_ provides to A, I mean the following. (Both these clauses, as well as the statement that _C_ screens off _B_ from A, are made relative to an evidential background. I’ll leave that as tacit in what follows.)

1. _B_ is evidence that A.
2. _B ∧ C_ is no better evidence that A than _C_ is, and _¬B ∧ C_ is no worse evidence for A than _C_ is.

Here is one stylised example, and one real-world example.

Detective Det is trying to figure out whether suspect Sus committed a certain crime. Let A be that Sus is guilty, _B_ be that Sus’s fingerprints were found at the crime scene, and _C_ be that Sus was at the crime scene when the crime was committed. Then both clauses are satisfied. _B_ is evidence for A; that’s why we dust for fingerprints. But given the further evidence _C_, then _B_ is neither here nor there with respect to A. We’re only interested in finding fingerprints because they are evidence that Sus was there. If we know Sus was there, then the fingerprint evidence isn’t useful one way or the other. So both clauses of the definition of screening are satisfied.

The real world example is fairly interesting. Imagine that we know Vot is an American voter in last year’s US Presidential election, and we know Vot is either from Alabama or Massachusetts, but don’t know which. Let A be that Vot voted for Barack Obama, let _B_ be that Vot is from Massachusetts, and let _C_ be that Vot is pro-choice. Then, somewhat surprisingly, both conditions are met. Since voters in Massachusetts were much more likely to vote for Obama than voters in Alabama, _B_ is good evidence for A. But, at least according to the polls linked to the state names above, pro-choice voters in the two states voted for Obama at roughly the same rate. (In both cases, a little under two to one.) So _C_ screens off _B_ as evidence for A, and both clauses are satisfied.

The Idea Behind JSE

When we think about the relation between _J_ and _E_, there are three conflicting pressures we immediately face. First it seems _J_ could be evidence for _p_. To see this, note that if someone else comes to know that _S_ has judged that _p_, then that could be a good reason for them to believe that _p_. Or, at the very least, it could be evidence for them to take _p_ to be a little more likely than they previously thought. Second, it seems like ‘double counting’ for _S_ to take both _E_ and _J_ to be evidence. After all, she only formed judgment _J_ because of _E_. Yet third, it seems wrong for _S_ to simply ignore _E_, since by stipulation, she has _E_, and it is in general wrong to ignore evidence that one has.

The simplest argument for JSE is that it lets us accommodate all three of these ideas. _S_ can treat _J_ just like everyone else does, i.e. as some evidence for _p_ without either double counting or ignoring _E_. She can do that because she can take _E_ to be _screened off_ by _J_. That’s a rather nice feature of JSE.

To be sure, it is a feature that JSE shares with a view we might call ESJ, or evidence screens judgments. That view says that _S_ shouldn’t take _J_ to be extra evidence for _p_, for while it is indeed some evidence for _p_, its evidential force is screened off by _E_. This view also allows for _S_ to acknowledge that _J_ has the same evidential force for her as it has for others, while also avoiding double counting. So we need some reason to prefer JSE to ESJ.

One reason (and I don’t think this is what anyone would suggest is the strongest reason) is from an analogy with the fingerprint example. In that case we look for one kind of evidence, fingerprints, because it is evidence for something that is very good evidence of guilt, namely presence at the crime scene. But the thing that we are collecting fingerprint evidence for screens off the fingerprint evidence. Similarly, we might hold that we collect evidence like _E_ because it leads to judgments like _J_. So the later claim, _J_ should screen _E_, if this analogy holds up.

JSE and Disagreement

My main concern here isn’t with any particular argument for JSE, but with the role that JSE might play in defending contemporary epistemological theories. The primary case in which I’ll be interested in concerns disagreement. Here is Adam Elga’s version of the *Equal Weight View* of peer disagreement, from his Reflection and Disagreement.

Upon finding out that an advisor disagrees, your probability that you are right should equal your prior conditional probability that you would be right. Prior to what? Prior to your thinking through the disputed issue, and finding out what the advisor thinks of it. Conditional on what? On whatever you have learned about the circumstances of the disagreement.

It is easy to see how JSE could lead to some kind of equal weight view. If your evidence that _p_ is summed up in your judgment that _p_, and another person who you regard as equally likely to be right has judged that ¬p, then you have exactly the same kind of evidence for _p_ as against it. So you should suspend judgment about whether _p_ is true or not.

But the distinctive role that JSE can play is in the clause about priority. Here is one kind of situation that Elga wants to rule out. _S_ has some evidence _E_ that she takes to be good evidence for _p_. She thinks _T_ is an epistemic peer. She then learns that _T_, whose evidence is also _E_, has concluded ¬p. She decides, simply on that basis, that _T_ must not be an epistemic peer, because _T_ has got this case wrong.

Now at first it might seem that _S_ isn’t doing anything wrong here. If she knows how to apply _E_ properly, and can see that _T_ is misapplying it, then she has good reason to think that _T_ isn’t really an epistemic peer after all. She may have thought previously that _T_ was a peer, indeed she may have had good reason to think that. But she now has excellent evidence, gained from thinking through this very case, to think that _T_ is not a peer, and so not worthy of deference.

Since Elga thinks that there is something wrong with this line of reasoning, there must be some way to block it. I think by far the best option for blocking it comes from ruling that _E_ is not available evidence for _S_ once she is using _J_ as a judgment. That is, the best block available seems to me to come from JSE. For once we have JSE in place, we can say very simply what is wrong with _S_ here. She is like the detective who says that we have lots of evidence that Sus is guilty&emdash;not only was she at the crime scene, but her fingerprints were there. To make the case more analogous, we might imagine that there are detectives with competing theories about who is guilty in this case. If we don’t know who was at the crime scene, then fingerprint evidence may favour one detective’s theory over the other. If we do know that both suspects were known to be at the crime scene, then fingerprint evidence isn’t much help to either.

So I think that if JSE is true, we have an argument for Elga’s strong version of the Equal Weight View, one which holds agents are not allowed to use the dispute at issue as evidence for or against the peerhood of another. And if JSE is not true, then there is a kind of reasoning which undermines Elga’s Equal Weight View, and which seems, to me at least, unimpeachable. So I think Elga’s influential version of the Equal Weight View stands and falls with JSE.

White on Permissiveness

In his 2005 _Philosophical Perspectives_ paper, Epistemic Permissiveness (PDF), Roger White argues that there cannot be a case where it could be epistemically rational, on evidence _E_, to believe _p_, and also rational, on the same evidence, to believe ¬p. One of the central arguments in that paper is an analogy between two cases.

Random Belief: _S_ is given a pill which will lead to her forming a belief about _p_. There is a ½ chance it will lead to the true belief, and a ½ chance it will lead to the false belief. _S_ takes the pill, forms the belief, a belief that _p_ as it turns out, and then, on reflecting on how she formed the belief, maintains that belief.

Competing Rationalities: _S_ is told, before she looks at _E_, that some rational people form the belief that _p_ on the basis of _E_, and others form the belief that ¬p on the basis of _E_. _S_ then looks at _E_ and, on that basis, forms the belief that _p_.

White claims that _S_ is no better off in the second case than in the former. As he says,

Supposing this is so, is there any advantage, from the point of view of pursuing the truth, in carefully weighing the evidence to draw a conclusion, rather than just taking a belief-inducing pill? Surely I have no better chance of forming a true belief either way.

But it seems to me that there is all the advantage in the world. In the second case, _S_ has evidence that tells on _p_, and in the former she does not. Indeed, I long found it hard to see how we could even think the cases are any kind of analogy. But I now think JSE holds the key to the argument.

Assume that JSE is true. Then after _S_ evaluates _E_, she forms a judgment _J_. Now it might be true that _E_ itself is good evidence for _p_. (The target of White’s critique says that _E_ is also good evidence for ¬p, but that’s not yet relevant.) But given JSE, that fact isn’t relevant to _S_’s current state. For her evidence is, in its entirity, _J_. And she knows that, as a rational agent, she could just as easily have formed some other judgment to _J_. Indeed, she could have formed the opposite judgment. So _J_ is no evidence at all, and she is just like the person who forms a random belief, contradicting the assumption that believing _p_ could, in this case, be rational, and that believing ¬p could be rational.

Without JSE, I don’t see how White’s analogy holds up. There seems to be a world of difference between forming a belief via a pill, and forming a belief on the basis of the evidence, even if you know that other rational agents take the evidence to support a different conclusion. In the former case, you have violated every epistemic rule we know of. In the latter, you have reasons for your belief, you can defend it against challenges, you know how it fits with other views, you know when and why you would give it up, and so on. The analogy seems worse than useless by any of those measures.

I think this analogy is crucial to White’s paper. Indeed, much of the rest of the paper consists of responses to objections to the argument from analogy made here. So I think if the analogy stands or falls with JSE, then the fortunes of White’s view on permissiveness are tied to those of JSE.

Christensen on Higher-Order Evidence

Finally, I’ll look at some of the arguments David Christensen brings up in his Higher Order Evidence. Christensen imagines a case in which we are asked to do a simple logic puzzle, and are then told that we have been given a drug which decreases logical acumen in the majority of people who take it. He thinks that we have evidence against the conclusions we have drawn.

Let’s consider a particular version of that, modelled on Christensen’s example of Ferdinand the bull. _S_ knows that ∀x(Fx → Gx), and knows that ¬(Fa ∧ Ga). _S_ then infers deductively that ¬Fa. _S_ is then told that she’s been given a drug that dramatically impairs abilities to draw deductive conclusions. Christensen’s view is that this testimony is evidence against ¬Fa, which I assume implies that it is evidence that Fa.

This looks quite surprising. _S_ has evidence which *entails* that _Fa_, and her evidence doesn’t rebut that evidence. It does, says Christensen, undermine her evidence for ¬Fa. But not because it undermines the entailment; it isn’t like the evidence gives her reason to believe some non-classical logic where this entailment does not go through is correct. So how could it be an underminer?

Again, JSE seems to provide an answer. If _S_’s evidence that ¬Fa is ultimately just her judgment that it is entailed by her other evidence, and that judgment is revealed to be unreliable because of her recent medication, then _S_ does lose evidence that ¬Fa. But if we thought the original evidence, i.e., ∀x(Fx → Gx) and ¬(Fa ∧ Ga), was still available to _S_, then there is a good reason to say that her evidence conclusively establishes that ¬Fa.

I’m not saying that Christensen argues from JSE to his conclusion. Rather, I’m arguing that JSE delivers the conclusion Christensen wants, and without JSE there seems to be a fatal flaw in his argument. So Christensen’s view needs JSE as well.

Conclusion

I’ve argued that Elga’s version of the Equal Weight View of disagreement, White’s view of permissiveness, and David Christensen’s view of higher-order evidence, all stand or fall with JSE. Not surprisingly, Christensen also has a version of the Equal Weight View of evidence, and, as Tom Kelly notes in his Peer Disagreement and Higher-Order Evidence (DOC), there is a strong correlation between holding the Equal Weight View, and rejecting epistemic permissiveness. Note that Rich Feldman, for instance, agrees broadly with Elga on disagreement, White on permissiveness and Christensen on higher-order evidence. Indeed, his work has been highly influential in all three of those fields. So these are not arbitrary selections from work of contemporary internalists.

I don’t, therefore, think it is a coincidence that these views stand or fall with JSE. Rather, I think JSE is a common thread to the important work done by various internalists on disagreement, permissiveness and higher-order evidence.

I also think that JSE is false, and is false for some fairly systematic reasons. But that is something that will have to wait for another post.