James Rachels, answered from the texts and cited to the page.
The moral rule against killing protects lives, not merely biological functioning. Being alive in the biological sense — the state of organismic metabolism, neural activity, basal physiological operation — is relatively unimportant; what matters is having a life in the biographical sense, where "a life" is the sum of one's aspirations, decisions, activities, projects, and human relationships.123
Mrs Florian, the Alzheimer's patient who has lost all semblance of human personality and whose vocabulary has shrunk to the words "fire" and "pain," is biologically alive but does not in the relevant sense have a life; her husband Hans Florian's act of ending her biological functioning (the case that opens The End of Life) was not the moral wrong the traditional doctrine of the sanctity of life declares it to be.
Karen Quinlan in the persistent vegetative state, Baby Jane Doe with congenital impairments severe enough to preclude development of biographical capacities, the Down's-syndrome infant in Anthony Shaw's nursery dying of dehydration and infection because surgery is being withheld — each is a case where biological life persists and biographical life does not, and where the traditional doctrine produces moral conclusions the case-by-case analysis cannot accept.4
The complement of the thesis is also true: some non-human animals have biographical lives in the relevant sense; killing them is morally objectionable for the same reason killing humans with biographical lives is. The thesis displaces the species-membership criterion (the "innocent humans" clause of the AMA position) with a criterion that tracks what actually makes life valuable, and applies the criterion consistently across humans and animals.5
Primarily they are facts about his history, character, actions, interests, and relationships. Once this distinction is made clear, we can see that there is a deep ambiguity in the notion of the value of life. Which is important to us-life in the biological sense, or life in the biographical sense? Plainly, the latter seems more important.Created from Animals - The Moral Implications of Darwinism, pp. 199–200
Having a Life To begin to make these claims plausible, we need to distinguish two notions that, in English, are expressed by similar words. We need to distinguish LIVES AND LIBERTY being alive from having a life.0 The former is fundamentally a biological notion: to be alive is (roughly) to be a functioning, self-preserving organism. The latter is, by contrast, a notion of biography rather than of biology.The Legacy of Socrates, pp. 89–90
The story of a person's life is not just an account of her status as a biological organism. It is the story of her history and character, her aspirations and disappointments, her activities and projects and personal relationships. A person's life, in this sense, includes all that she says and does and thinks.Can Ethics Provide Answers, pp. 73–74
If the traditional theory is not true, then in our society many decisions concerning life and death are being made on unsound grounds, and the law concerning such matters is badly in need of reform. I believe that the traditional view is mistaken at almost every point. The maze of distinctions'on which it is based cannot withstand analysis. Much of this essay is a defence of that judgement.The End of Life - Euthanasia and Morality, pp. 5–6
He had lost part of his jaw, his upper lip, his nose, and his left hand. Growths had been removed from his right arm and two fingers from his right hand. He was left blind, slowly deteriorating, and in agony of body and soul. The pain was constant; at its worst, he could be seen lying in bed with teeth clenched and beads of perspiration standing out on his forehead.The End of Life - Euthanasia and Morality, pp. 32–33
The biographical-life thesis is the criterion that The End of Life offers to replace the traditional sanctity-of-life doctrine. It has been engaged by the personhood-criterion tradition in bioethics (Mary Anne Warren, Michael Tooley) which develops adjacent criteria, and by Peter Singer's preference-utilitarian framing in Practical Ethics which arrives at similar practical conclusions through a different theoretical apparatus. It has been attacked by the sanctity-of-life Christian tradition (Ramsey, the Catholic moral-theology framing) on grounds that the criterion devalues biological life as such, and by the disability-rights tradition on grounds that it devalues the lives of severely cognitively impaired humans. The position defends the criterion against the sanctity-of-life charge as the displacement of species-membership reasoning the traditional doctrine implicitly relies on, and engages the disability-rights critique on the substantive ground that "biographical life" is not the same as "cognitively typical life" — the criterion does not require the capacities of the average adult, only that the individual have a life in the relevant sense, which most disabled persons clearly do. The hard cases are acknowledged as hard cases the analytical framework can examine.