1. "Human being" is vague because it's used colloquially to describe two different things (biology and personhood). In your use of the term, you yourself are an example of this.
Human being is not vague, you are trying to make it so but it is not. Somehow you have separated human being from person based on your subjective assertion of some arbitrary developed brain activity.
A human being is a man, woman, or child of the species
Homo sapiens, distinguished from other animals by superior mental development, power of articulate speech, and upright stance. That's the dictionary definition. The definition is compared to the stark difference of non Homo Sapien animals.
The question of when a human being begins is strictly a scientific question and not philosophical but answered by embryologists.[/QUOTE
2. Science did not define "human being" since the phrase has been around since the late 1600's.
Yes they do and we have a lot more 'science' on the matter today:
https://www.princeton.edu/~prolife/articles/wdhbb.html
3. Personhood is used to establish rights rather than biology.
Personhood is the quality or condition of being an individual person. That's the basic definition. So we go back to when does one become a distinct individual person? Science has an answer for that and it is conception.
What you are referring to? Moral personhood, legal personhood or Constitutional personhood? Matters not as none of the aforementioned defines one as a human being. They are philosophical terms which thousands of opinions exist. So I ask, who is right in their assessment of 'personhood?'
5. It's logical to take the position that brain dead bodies aren't persons. Therefore, it's also logical to hold the position that brain functions should be used in determining personhood.
Who determines 'brain dead?' That's right medical doctors who are scientists. You trust them to determine brain dead but not the start of a human being.
Plus your logic is flawed. Brain dead means dead. It is not a live human being developing. Before pulling the plug on a brain dead person an advanced directive is needed or permission from the family. Some form of consent knowing the wishes of the brain dead person. Embryos get no vote. Not a very good comparison.
Now, what group, organization or magisterium gets to decide what constitutes brain activity commensurate with 'personhood?' Who is going to stand in judgment to decided who is a person or not? Government? The medical profession? Philosopher kings? Who exactly and when? That would have to be determined since you want to ignore science altogether and deny at conception we have a new human being which is developing towards being eventually a full adult. So we have from conception to full adult a process of development and you must now determine the 'when' and 'why.' I already gave you quite a few examples, using your approach, of those who don't see newborns as 'persons' because their brains are not developed for higher learning. Are they wrong and why are they wrong?
6. First semester fetuses don't display the higher brain functions that we associate with personhood. Therefore, first trimester fetuses aren't persons.
That is your subjective analysis based on a preconceived conclusion. A flawed conclusion at it.
Again, your 'logic' led many, many others to much different conclusions. Some not considering Downs pre-born and newborn as persons. Some not considering newborn babies up to toddler as 'persons.'
We truly do live in a brave new world. Where our society is now trying to define human life by eugenics.