That being said, I don't want to talk about the long run at all, because we don't make decisions for the long run. For instance, you could decide to have a bacon omelette for dinner today and then stay veg*n for the rest of your life, and the argument that you attribute to me wouldn't work in that case, although I would urge you to not eat the bacon omelette.
The point is that given the way these probabilities add up, not only wouldn't that work for a single bacon omelette, it wouldn't work for a lifetime of bacon omelettes. They're either all harmful or all non-harmful.
Therefore, even if you only assign the "cows are sentient" or "theory X is true" outcomes a probability of 20%, the bad outcomes are so bad that we shouldn't risk them.
Your reasoning doesn't depend on the exact number 20. It just says that the utility of the outcome should be multiplied by its probability. If the probability was 1% or 0.01% you could say exactly the same thing and it would be just as valid. In other words, your reasoning proves too much; it would imply accepting Pascal's Mugging. And I don't accept Pascal's Mugging.
The point is that given the way these probabilities add up, not only wouldn't that work for a single bacon omelette, it wouldn't work for a lifetime of bacon omelettes. They're either all harmful or all non-harmful.
I know. Are you implying that we shouldn't maximise expected utility when we're faced with lots of events with dependent probabilities? This seems like an unusual stance.
Your reasoning doesn't depend on the exact number 20... If the probability was 1% or 0.01% you could say exactly the same thing and it would be just as valid.
My reasoning...
I'm currently unconvinced either way on this matter. However, enough arguments have been raised that I think this is worth the time of every reader to think a good deal about.
http://nothingismere.com/2014/11/12/inhuman-altruism-inferential-gap-or-motivational-gap/