Wednesday, October 27, 2010

Newcomb and medical Newcomb

Leaving aside Egan's counterexamples to causal decision theory, which I need to think about more before I say anything, it seems to me that one way to view the debate between the causalist and the evidentialist is whether one should make the medical Newcomb cases or the standard Newcomb cases drive one's intuitions. Here is a reason to make the medical Newcomb cases drive one's intuitions: medical Newcomb cases are much closer to reality. While it is hard to imagine reliable prediction of choices, it is much easier to imagine factors that simultaneously affect behavior and other outcomes. Our intuitions are more reliable about cases that happen in worlds closer to ours, and medical Newcomb cases happen in worlds closer to ours.

In fact, medical Newcomb cases might very well happen in the actual world. Imagine the following story. There is a disease D that in its late stages horribly painful, incurable and fatal. In early stages it is very hard to detect. It has only one cure: regular jogging in early stages can cure it 20% of the time. This has resulted in the following evolutionary adaptation. Early stage sufferers from D are more likely to jog regularly. I am now trying to decide between regular jogging and regular biking. Suppose both are equally healthy in all other respects. Surely I should say to myself that jogging has the added benefit of reducing the risk of D, and go ahead and jog. But if I am an evidentialist, and the correlation between jogging and early stage D is strong enough, it can be the case that the expected value of jogging is lower than the expected value of not jogging. So evidential decision theory then leads me to act contrary to a very sensible evolutionary adaptation by avoiding the one and only thing that could cure D, and to do this in the name of avoiding D. That is perverse. (There is a way in which this case is a kind of reverse of the Egan cases. This fact may be important for figuring out what to do with the Egan cases.)

(Regular jogging is somewhat complex here, because it's a policy or habit rather than single act. We can replace jogging with eating unripe pears, or something like that.)

It would be quite unsurprising if there were such evolutionary adaptations. It would be surprising if there were actual Newcomb cases. It is more important for our decision theory to give us what are intuitively the right answers in these medical cases than in the standard Newcomb cases.

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