Let’s say someone is a person if and only if he possesses self-awareness, consciousness, rationality, the ability to communicate, and so on. Call that the standard view. The standard view is found in Singer, Glover, Tooley, Lowe, Williams, McMahan, and Parfit and goes at least as far back as Locke. According to the standard view, the property of being a person confers a special moral status on those who instantiate it. Only persons have the full profile of moral rights, so their lives have a moral protection that is not afforded to non-persons.
If I were a person, then all of the following would be true:
1. It is impossible that you might have killed me as a toddler.
2. It is impossible that I might exist and lose my rationality, self-awareness, ability to communicate, etc.
3. It is impossible that I might exist and lose consciousness.
4. It is impossible that I might exist and regain consciousness.
5. It is impossible that I might exist and miraculously recover from Alzheimers.
All of (1) – (5) are plainly false. I am therefore not a person. Despite not being a person–you’re not one either, by the way–I have moral rights, and so do you. I conclude that the basis of moral rights is not personhood.