“Why does a potential human being have a right to life that is equal to an existing life”
And just like that…the personhood argument. Remember what I said about every abortion argument boiling down to denying (or affirming) the moral value of a fetus?
Of course if I’m going to be rude, I’ll take your statement literally and point out that fetuses are categorically both humans AND existing life so your attempt at distinction fails.
Now what you probably mean is “why does an undeveloped human have the same right to life as a fully grown human”. It comes from a descriptivist argument of the wrongness of killing. If it is not permissible to kill adult people on the basis of future conscious experience, then this also applies to fetuses because they too have future conscious experiences.
Now the problem is showing that future conscious experience is the core reason for the wrongness of killing. It’s descriptively very powerful, it accounts for the permissibility of letting brain-dead individuals die (or even actively killing them), the impermissibility of killing temporarily unconscious persons, and the impermissibility of active killing of temporarily suicidal persons (the later problem is also fatal to Boonin’s cortical organisation argument, as it is not the current desires of an individual that we have a moral imperative to satisfy but rather an idealised person with desires considered rational. Boonin’s argument relies on fetuses not having desires to continue living, but this is simply special pleading; a person lacking desires would not permit them to be killed anyway because of the aforementioned idealised rational desires).
Now we have a moral principle that accounts for all of these clearly immoral acts. When we apply it to abortion, we find that it is also not permitted. So do we reject this principle in favor of all the other principles that allow abortion along with the other active killing that we agree is immoral?
Or do we consider that abortion is a complex decision that is clouded by personal preference, desire for convenience, and ignoring empirical facts in favor of prima facie evaluation? (i.e fetuses don’t look or act human, therefore they must not be, contrary to all deeper evaluation).
In other words, it seems highly plausible that our superficial feelings about abortion are NOT morally relevant, and the moral principle that does correctly describe the morality of other active killing is also correctly describing the morality of abortion as well.
Note that it is not necessary for the right to life of a fetus to be equal to an adults to make abortion immoral. It simply has to be sufficiently strong enough to prohibit in convenience cases. Just like how dogs don’t have to have the same moral value as humans to prohibit killing them for fun, it just needs to be sufficiently high to outweigh any moral value of the fun.
“Where do rights come from, in your view?”
I already addressed this when talking about determining moral principles. They come from our intuitions about what is wrong and what is logical reasoning.