Brain stem activity does constitute personhood (morally, not legally). The brain is made of nerves. Nerves, in turn, are made of cells, and cell division is what drives fetal development. Some level of consciousness must exist from conception, therefore.

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"I am a Tory Anarchist. I should like every one to go about doing just as he pleased- short of altering any of the things to which I have grown accustomed." (Max Beerbohm)