Quote:
Originally Posted by Jibninjas
So a sperm cell will one day become an adult by itself if left alone?
A sperm cell will become a zygote in the right environment, and die in the wrong environment. A zygote will become an embryo (let's skip some steps) in the right environment, and die in the wrong environment. An embryo will become a fetus in the right environment, and die in the wrong environment. A fetus will become a baby in the right environment, and die in the wrong environment. A baby will become a child in the right environment, and die in the wrong environment.
I think "child" pretty much represents the highest qualitative level in this chain. Most any environment that can support an adult can also support a child (though in most cases the child will bear an increased risk of mortality).
Every step down the chain involves an extra ingredient necessary for maturation:
A child needs food, water, air, shelter, stable temperature, and adequate stimulation available in its environment.
A baby needs all of the above, and a baby needs these resources delivered to it (it is not able to gather them from its own environment).
A fetus needs all of the above, needs its breathing done for it, and needs highly specific physical conditions resembling those of the womb.
An embryo needs all of the above, needs highly specific chemical conditions resembling those of the womb, and needs many organ systems to be externally supported.
A zygote doesn't breathe or have organ systems, otherwise it needs all of the above. A zygote also needs a complex series of signals, materials, and ready-made protein structures activating at the rights times and in the right places.
A sperm cell needs all of the above, plus an extra set of haploid genetic material.
There is no place on this chain that has fundamental significance. Nothing below the level of a child can survive "on its own" in any environment, everything from baby on down needs significant external support. That doesn't represent a legitimate logical basis for differentiation, unless we conclude that people don't have value until they're 2 years old. There are some potentially logical reasons to draw the line at conception (for example, valuing diploid DNA above haploid, though it seems silly to me). But the "by itself if left alone" approach doesn't fly at all.
Personally I think capacity for suffering is the human capacity that accords value to people, and neuroscience says the capacity for suffering is likely related to complex brain activity. Development of the frontal cortex, then, is probably the most rational "line" for me to draw if such a black-and-white approach needs to be taken.