Andy Egan argues:

The Smoking Lesion

Susan is debating whether or not to smoke. She knows that smoking is strongly correlated with lung cancer, but only because there is a common cause – a condition that tends to cause both smoking and cancer. Once we fix the presence or absence of this condition, there is no additional correlation between smoking and cancer. Susan prefers smoking without cancer to not smoking without cancer, and prefers smoking with cancer to not smoking with cancer. Should Susan smoke? Is seems clear that she should. (Set aside your theoretical commitments and put yourself in Susan’s situation. Would you smoke? Would you take yourself to be irrational for doing so?)

Causal decision theory distinguishes itself from evidential decision theory by delivering the right result for The Smoking Lesion, where its competition – evidential decision theory – does not. The difference between the two theories is in how they compute the relative value of actions. Roughly: evidential decision theory says to do the thing you’d be happiest to learn that you’d done, and causal decision theory tells you to do the thing most likely to bring about good results. Evidential decision theory tells Susan not to smoke, roughly because it treats the fact that her smoking is evidence that she has the lesion, and therefore is evidence that she is likely to get cancer, as a reason not to smoke. Causal decision theory tells her to smoke, roughly because it does not treat this sort of common-cause based evidential connection between an action and a bad outcome as a reason not to perform the action.

Egan’s argument is basically that either she has the lesion or she does not, and she can make no difference to this, and so apparently she can make no difference to whether or not she gets cancer. So if she feels like smoking, she should smoke. If she gets cancer, she was going to get it anyway.

Answering Egan’s question, if there was a strong correlation like this in reality, I would think that smoking was a bad idea, and would choose not to do it.

We can change the problem somewhat, without making any essential differences, such that every reasonable person would agree.

Suppose that every person is infallibly predestined to heaven or to hell. This predestination has a 100% correlation with actually going there, and it has effects in the physical world: in some unknown place, there is a physical book with a list of the names of those who are predestined to heaven, and those who are predestined to hell.

But it has nothing to do with the life you live on earth. Instead, when you die, you find yourself in a room with two doors. One is a green door with a label, “Heaven.” The other is a red door with a label, “Hell.” The doors do not actually lead to those places but to the same place, so they have no special causal effect. You only end up in your final destination later. Predestination to heaven, of course, causes you to choose the green door, while predestination to hell causes you to choose the red door.

You find yourself in this situation. You like red a bit more than green, and so you prefer going through red doors rather than green ones, other things being equal. Do you go through the green door or the red door?

It is clear enough that this situation is equivalent in all essential respects to Egan’s thought experiment. We can rephrase his version:

“Susan is debating whether or not to go through the red door. She knows that going through the red door is perfectly correlated with going to hell, but only because there is a common cause – a condition that tends to cause both going through the red door and going to hell. Once we fix the presence or absence of this condition, there is no additional correlation between going through the red door and going to hell. Susan prefers going through the red door without going to hell to not going through the red door without going to hell, and prefers going through the red door with going to hell to not going through the red door with going to hell. Should Susan go through the red door? Is seems clear that she should. (Set aside your theoretical commitments and put yourself in Susan’s situation. Would you go through the red door? Would you take yourself to be irrational for doing so?)”

It should be clear that Egan is wrong. Don’t go through the red door, and don’t smoke.