So, for example, some people take the view that human beings become “persons,” and acquire rights, only when they acquire the capacity for abstract mental functioning, and cease to be persons with rights when they lose this capacity. If “capacity” is taken to mean immediately exercisable capacity (as it usually is on this view), then it is possible to allow abortion, research that destroys human embryos, and euthanasia of the permanently comatose and persistently vegetative.

There are, however, difficulties with this view. …

Second: The capacity for abstract mental functioning varies continuously. But it is impossible to identify, without arbitrariness, the minimum level one must have to enjoy rights. It is also impossible to explain why people who have more of the quality should not be regarded as greater in worth, dignity, and rights than people who have less of it. (This is true, and necessarily true, of any of the qualities generally proposed as the conditions of worth: self-awareness, rich interactions with others, the ability to experience pain and pleasure, etc.) The notion that all human beings are created equal becomes a self-evident lie.
–Ramesh Ponnuru, The Party of Death (more when I finish the book)