There are a lot of explanations of consequentialism and utilitarianism out there, but not a lot of persuasive essays trying to convert people. I would like to fill that gap with a pro-consequentialist FAQ. The target audience is people who are intelligent but may not have a strong philosophy background or have thought about this matter too much before (ie it's not intended to solve every single problem or be up to the usual standards of discussion on LW).
I have a draft up at http://www.raikoth.net/consequentialism.html (yes, I have since realized the background is horrible, and changing it is on my list of things to do). Feedback would be appreciated, especially from non-consequentialists and non-philosophers since they're the target audience.
I don't know how to derive my impression from first principles. So the answer has to be: because my moral intuitions tell me to do so. But they only tell me so in this particular case -- I don't have a general rule of disregarding future information.
Ok. I will try to articulate my reasoning, and see if that helps clarify your moral intuitions: a "life" is an ill-defined concept, compared to a "lifespan." So when we have to choose one of two individuals, the way our choice changes the future depends on the lifespans involved. If the choice is between saving someone 10 years old with 70 years left or someone 70 years old with 10 years left, then one choice results in 60 more years of aliveness than the other! (Obviously, aliveness is not the only thing we care about, but this is good... (read more)