A subject’s justification for a belief is not stronger than a second subject’s justification for the same belief, if their respective justifications are prone to being equally well defeated by the same defeaters (Defeat).
His objection assumes that veridical perceptual experience and subjectively indistinguishable hallucination are equally well defeated by the same defeaters because they are subjectively indistinguishable. So, if his objection is sound, it shows that if two conscious experiences are indistinguishable, the reasons they provide for your beliefs are equally strong and these experiences will justify the same beliefs to the same degree.
If we read Defeat as saying that the reasons provided by two mental states are of the same strength if they are defeated by some common defeater, the principle implies that the justification you have for believing that you will win this hand of poker when you know that you have four aces is no better than the justification you have if you have a pair of aces because you know you cannot justifiably believe you will win if you peek and see that someone is holding a royal flush. If Defeat has any plausibility, we have to assume that the justification provided by two states is equally strong if these justifications are liable to defeat by all the same defeaters. I still think the Defeat principle is problematic, but it will take a bit of work to show that this is so.
Consider now two theses about indiscriminability and justification:
TransitivityI: (x)(y)(z)[(Ixy &Iyz) --> Ixz)]
TransitivityJ: (x)(y)(z)[(Jxy &Jyz) --> Jxz)]
Read 'Ixy' as x and y are indiscriminable for you and 'Jxy' as x and y justify the same beliefs to the same degree.
Arguably, TransitivityI is false. Suppose a, b, and c are perceptual experiences. Suppose a is indiscrimimable from b, b is indiscriminable from c, but a is discriminable from c. Just think of the experiences you have when you look at paint chips. If these chips differ only slightly, you might be unable to distinguish the first from the second and the second from the third even if the first and third can be told apart on the basis of experience. What goes for the chips goes for the perceptual experiences of the chips. It seems that TransitivityJ is true. For it to be false, there would have to be some proposition, p, such that the degrees to which a and c justified belief in p differed even though both a and c justified belief in p to the same degree that b does. This is impossible.
With this in mind, I shall argue that the principle Conee is committed to is mistaken. Conee’s principle states:
(1) (x)(y)(Ixy --> Jxy)
I think we should assume that:
(2) (x)(y)(~ Ixy --> ~ Jxy)
The justification for (2) is that in discriminating between two things, you can know that these two things are distinct. You will have stronger reasons for believing that you are undergoing a while undergoing a than you will have for believing that you are undergoing some experience you can knowingly discriminate from a (e.g., c). If a is indiscriminable from b, b is indiscriminable from c, but you can discriminate between a and c, (1) entails that a and b justify the same beliefs to the same degree. It also entails that b and c justify the same beliefs to the same degree. It follows by TransitivityJ that a and c justify the same beliefs to the same degree. But, this contradicts the further assumption that a and c are experiences that you can discriminate between given the further assumption that (2) is correct. The most obvious way to avoid this contradiction is to deny Conee’s principle, (1).
Recall that Conee’s objection to McDowell was that McDowell’s view implied that it is possible for indistinguishable states to provide different reasons for belief. His objection to this view assumed that indistinguishable states can be defeated by precisely the same considerations and that states that can be defeated by precisely the same considerations cannot offer different reasons. Now, I think we can see that these assumptions cannot both be correct. Either the reasons provided by two indistinguishable states are not defeated by the very same considerations or the reasons provided by two states can be defeated by the same considerations even if these states provide different reasons.
I’m suspicious of his defeat principle. Can reasons defeated by all the same defeaters differ in strength? If reasons behave like boxers they can. Nobody can defeat Mustard in a boxing match. Apart from Mustard, nobody can defeat White or Plum. White and Plum cannot box against one another because they share gloves. Plum and Green do not box against one another because Plum and Green share trunks. You need a boxing match to bet on because you have debts that no non-betting person can repay in time. If you do not know who the opponent will be and the rules require that you bet on the fighter you send into the ring, you have better reason to send White into the ring than Plum. It is true that they would both be defeated by the same opponent. You will lose if Mustard is waiting for your fighter in the ring. If you were to send White to the fight and Green was the opponent, you would win. If you were to try to send Plum into the fight and Green were the opponent, the fight could not take place and you could not win your bet. If the opponent is neither Mustard nor Green, you win. The lesson seems to be that the comparative strength of two reasons is not fixed by facts about what defeats these reasons. Not if reasons are like boxers. I think some reasons are like boxers. Reasons to bet on boxers are reasons. They seem to behave a bit like boxers.