It still bears the question at what point are we considered alive in philosophy. The terms you used can clearly be applied to a young child or a person in a coma for instance.
It is the concept most people use when defending abortion but I never heard it exactly as you put it. I am not questioning it but wish to understand it.
That's precisely the point.
First of all, there are varying views on this. But the point is biology alone can't answer it (and I'm not saying this for your benefit).
The difference between a young child and a fertilized egg - or a fetus for that matter - is that it can live on its own outside of the womb. Granted, the child will need food and shelter, but its not a single cell or mass of cells with no nervous system or anything else that could potentially denote personhood.
After a child can survive outside of the womb, and particularly after it is born, it is without question a human being despite its possible deformities. The human being, whether functional or non-functional is a symbol of the great potentials of humanity and we treat them with all the dignity and respect that a human deserves. This includes the right to die as one desires.
The fertilized egg, zygote, and embryo have basically no potential as a symbol of the great potentials of humanity and we have no need to treat them with all the dignity and respect that a human deserves. There is no chance for survival outside of the womb, and no chance for personhood, human expression, or achievement.
