Neither your criterion, nor the pain criterion, are meaningful unless you believe in supernatural things like gods or souls. Believing in the supernatural witho

Neither your criterion, nor the pain criterion, are meaningful unless you believe in supernatural things like gods or souls. Believing in the supernatural without evidence is not what I would call rational - if you believe that, why not also believe in pixies in your garden, or Russell's Teapot? Humans are just self-replicating collections of atoms that emerged through chance and evolution, and pain is just an emergent phenomenon formed by electrical impulses travelling over neurons to signal things that we evolved to avoid. So there is no absolute moral reason to protect the coherence of a particular collection of atoms over another, or to favour a doctrine of hedonism over pain. Ultimately, to optimise for something, we need a criterion on which to optimise. For the above reasons, I consider preserving human biodiversity, or some kind of hedonic pleasure-pain balance as a fairly meaningless thing to pick as a fundamental thing to optimise for in the scheme of things (although it may emerge as a result of another more meaningful optimality criterion). I consider the diversity of information / knowledge (that is, information that encodes something true about the world, not just random entropy) recorded in the universe in a form that can be processed by intelligently acting constructions to be a more fundamental thing to optimise for. Obviously, picking what to optimise for is completely arbitrary - it is used to justify everything, but it is like an axiom - there is nothing else to justify it in terms of. Optimising for such knowledge means protecting the intellectual independence of constructions that create, process, and support the construction of such knowledge. Since this is primarily humans at the moment, this means ensuring that humans are not oppressed. Pain and fear of death are means of oppression of humans, and so it makes sense to ensure that living, thinking humans capable of knowing fear and feeling oppressed do not have to fear that they will be subjected to arbitrary death or pain for their intellectual activities. One way to do this is to strictly enforce a no humans killing humans older then some certain amount of time after conception. If the amount of time after conception is too high, an age at which the person can know that they are at risk of death and fear it, then it will be oppressive. However, an embryo or fetus is very unlikely to be know that abortion exists, or to do anything differently to avoid it, and therefore abortion cannot be oppressive like murder or torture is. Anyone capable of knowing about abortion is too old to be killed deliberately by it.