I expect to write a few posts on this topic, so here's a quick overview and introduction:
We should be wary of bringing about terrible outcomes, in face of empirical uncertainty. I assume it's terrible to kill innocent persons (i.e. conscious, rational beings with goals for the future). So if there was a 10% chance that fetuses had mature psychological capacities, that would - I take it - count as a decisive reason against getting an abortion (in typical circumstances). But what about merely normative uncertainty? Suppose we're sure that fetuses have minimal mental lives of type F, but we nonetheless grant a 10% chance that killing a type-F is as morally bad as killing a mature person. What weight should we grant this moral uncertainty in our practical reasoning? Here are three possible answers:
(I) Full weight: The two kinds of uncertainty are normatively equivalent.
(II) Some weight: Normative uncertainty should count for something, but not so much as a corresponding empirical risk.
(III) No weight: Merely normative risks have no place in practical reasoning.
The 'full weight' view has some bizarre implications. For example, if you think shooting a gun out the window has a 1% chance of killing someone, and allow a 1% chance that masturbation is as morally bad as killing someone, then - according to (I) - you should be indifferent between the two actions. (Helen suggests that this just goes to show you shouldn't grant even that much credence to the loony moral view. That seems a good response; I'll probably return to it later.)
I previously suggested that we should distinguish between probabilistic reasons - i.e. real reasons that derive from the modal fact, X, that some outcome Y is epistemically possible - and probabilities of reasons, i.e. the mere epistemic possibility that a certain fact Z even qualifies as a reason in the first place.
One may object: why can't the epistemic possibility of a normative proposition [Y] constitute a real reason? I'll explore this more in the next post: Evidence, Reasons and Normative Doubts.