It s not as easy as it might first appear. Most people recognize two plus two equals four. It s a given until someone comes along and questions it. Then it takes several pages of mathematical notation to prove the equation. In the same way the personhood of a human is usually assumed.
Every human has an intrinsic value which is equal to every other human.
This is a truth our American Founding Fathers accepted as intuitively obvious. Questions about personhood are usually only raised when someone wants to do something to an individual they would not want done to a person. Defining personhood, then, becomes crucial in determining who we value and protect. The Fourteenth Amendment says the state shall not deprive any person of life without due process of law. Aside from the fact that killing an innocent person is unconstitutional it is also morally repugnant to decent folks.