Manifestaciones clínicas

A first natural thought is that shouldF OE is relativized to FOE, while shouldHOE

is relativized to both the FOE and the HOE, meaning that, if we model bodies of evidence as sets of possible worlds, it’s relativized to FOE∩HOE, the intersection of the two. This fits how Feldman (2005) thinks about how one should react to HOE: it is just more evidence, which is added to the evidence one already possesses (one’s FOE). What one shouldHOE—or for Feldman, simply should—believe is

what one should believe in light of this more informed body of evidence.

A problem with this approach, however, is that the support relation between FOE and P will, at least in some cases, also hold between FOE ∩HOE and P. This is problematic where the relevant HOE is meant to undermine FOE. In such scenarios, the same doxastic attitude to P will be licensed (i) by the support relation between FOE and P and (ii) the support relation between FOE ∩HOE andP, where we actually want different doxastic attitudes to be licensed by these relations.

This is most obviously the case where one’s FOE entails the target proposition, as in Logical Reasoning. FOELR ∩ HOELR also entails ¬F a. So if it is the

entailment relation between FOELR and ¬F a that makes it the case that I

shouldF OE believe¬F a, then it seems that in light of FOELR ∩HOELR, I should

also believe ¬F a.

The problem is not limited to deductive support relations. Christensen (2010: 197) claims that if there exist explanatory connections between FOE and a target proposition P, “[t]hese connections [...] do not depend on any claims about me, and the new information I learn about myself [i.e., the HOE] does not break these connections”. Christensen does not provide any arguments for his claim, so let’s consider Abductive Reasoningto have a closer look at it.

The relevant bodies of evidence and the target proposition in the case are:

• FOEAR: Patient has symptoms X

• HOEAR: I have been up for 36 hours and the diagnoses of sleep-deprived

doctors can be right just as well as wrong

FOEAR supports Arthritisin virtue of the fact that Arthritis is the best expla-

nation for the patient having symptom X. Christensen is certainly right that FOEAR has this support relation toArthritis, irrespective of whether HOEAR

is true or not. The fact that I am sleep-deprived seems to be irrelevant to the medical question of whether Arthritis is the best explanation for the patient having symptomsX.

However, it is not clear that just because HOEAR does not bear on the

abductive support relation between FOEAR and Arthritis, it is guaranteed that

the same support relation holds between FOEAR ∩HOEAR andArthritis. Take

a case where you have FOE that all of the 100 swans you have observed so far have been white. This body of FOE gives strong enumerative inductive support to the proposition<All swans are white>(or at least the proposition<most swans are white>). Then you gather more FOE, which consists of an observation of 100 black swans. While the fact that you’ve observed 100 black swans doesn’t make a difference to the support-relation between your previous body of FOE and the proposition that all swans are white, the intersection of this new FOE and your previous FOE does obviously not have the same support relation to the proposition<All swans are white>.

Thus, even if we pool a body of FOE with another body of evidence that does not bear on the evidential relation the original FOE has to the target proposition

P, their intersection might have a different support relation toP. To check which support relation holds betweenArthritisand FOEAR ∩HOEAR, let’s examine

whether Arthritis is the best explanation for FOEAR ∩ HOEAR. Whether it

is depends on the alternative possible explanations. For the sake of simplicity, let’s assume there is only one other possible disease—lupus—which could have caused symptomsX, but that the symptoms are more typical for arthritis than lupus. Therefore, Arthritis is a better explanation for symptoms X than the proposition that the patient has lupus (Lupus). Now, if we add evidence that I was sleep-deprived when concludingArthritis (HOEAR) to my evidence that

patient has symptoms X (FOEAR), is this joint body of evidence (FOEAR ∩

HOEAR) better explained by Arthritis orLupus? I suggest we answer this by

reasoning in the following piecemeal fashion:

First, neither ArthritisnorLupusis a better explanation of HOEAR, i.e., the

diagnoses can just as well be wrong as right. If anything, neither of them explains this. Second, Arthritisexplains the FOEAR-part of the joined body of evidence

better than Lupus does. Combining these two factors, we can conclude that

Arthritisexplains the joint body of evidence better thanLupusdoes. Hence, this joint body of FOE and HOE licenses believing Arthritis, rather than suspending judgement on it, just as the original FOE did.

I conclude that in all the three cases described above, the joint body of evidence resulting from adding the HOE to the FOE has the same support relation with the target proposition as the FOE alone has. So, Approach 1 fails.