Some think that the life of a human being who has permanently and irreversibly lost consciousness has no value. Here are three arguments against tying human value and human dignity to consciousness.
Argument 1: Leibniz and Freud have taught us that much of our mental life is unconscious. If we just look at a typical person's conscious episodes what we get is a disconnected life, a series of short film clips, rather than the rich story that a typical human life is. It would be strange, then, to make the conscious life be the sole locus of value. This argument is there just to move one's intuitions away from an excessive focus on consciousness. It won't, for instance, be relevant in the case of brain damage so severe that there is good reason to think there are no unconscious mental processes (though in practice it does caution one; we know that medical personnel can be mistaken about whether a patient is conscious, and it seems to be even more difficult to determine whether there are unconscious mental processes).
Argument 2: Some living things, like trees, exhibit metabolic activity. Other living things, like earthworms, exhibit significant movement. Other living things, like geckos (I assume), exhibit conscious awareness. Yet others, like dogs, exhibit significant and flexible problem-solving skills. And others yet, of which the only example we are empirically sure are humans, exhibit the kind of sophisticated intellectual functioning and interaction that is characteristic of persons. But the later entries in this list also exhibit the activities of the earlier ones. Earthworms not only move, but also metabolize. Geckos not only are consciously aware, but also move and metabolize. Dogs not only solve problems, but are conscious, move and metabolize. And humans do all of these—and exhibit sophisticated cognition on top of it. The life of a tree, a worm, a gecko and a dog has value, and the good that is found in each of these is found in the typical life of a human. Not all of these goods require consciousness: the good of metabolism and movement is present in many animals without, as far as we know, consciousness. Thus the life of a human who does not exhibit consciousness nonetheless exhibits a number of other goods. To deny this is basically to deny that humans are animals, or to take the implausible view that the life of a tree or a worm has no value.
Argument 3: Consider the attitude one might have towards someone that one loves who has fallen dreamlessly asleep—say, one's child or one's spouse. One may fondly kiss the beloved's head, recognizing the beloved's present value—fondness always involves an element of taking the beloved to have value. If the value of humans essentially requires consciousness, there is either a mistake here or else the value is entirely constituted by the expected future consciousness. It is implausible to say that a mistake is being made, so let us consider the future-consciousness hypothesis. Suppose that the beloved is going to be executed by a tyrant as soon as she about to regain consciousness. Then there is no future consciousness (except in the afterlife, and I do not think the attitude depends on beliefs about the afterlife). But the tragic absence of a future consciousness does not make one less fond—it does not make one value the person less—but the very opposite. Nor is one's attitude as it is towards a corpse. In the case of the sleeping person who will be executed, one dreads and mourns a future loss; in the case of the corpse, one mourns an already present loss.
Final remarks: The above establishes a weak conclusion: that there is intrinsic value in an unconscious human life. One might think that this weak conclusion avails little. But I think it establishes one thing: It is a mistake to think that one can be bestowing a good on an unconscious patient by killing her. An unconscious patient is not suffering. The evils that have befallen her are evils of privation (maybe the evil of suffering is also an evil of privation, but that is more controversial)—she lacks consciousness, speech, complex two-way interaction, etc. But she still exhibits the kinds of goods that oak trees exhibit. And to kill her is to deprive her even of these goods. (A religious person might say: "It will hasten her happiness in the next life, and that is of value." But rarely do we know that a person's afterlife will be free of suffering. Besides, the hastening is not much of a benefit, because when one is unconscious, one is not waiting. According to her subjective time, she will get the goods of the afterlife just as quickly if she is killed now as when she is allowed to live for another ten years of unconsciousness.)
One might think that it is an indignity for a human to be active only at the level of plants. That, I think, is too high a view of humans. We all begin with a life of purely metabolic activity after conception, and most of us end with a life of purely metabolic activity (if only for a few seconds).
An important question (Trent Dougherty asked me about this today), but one that is not required for my weak conclusions above, is whether the metabolic life is intrinsically more valuable in the human than in the oak tree. The answer is, I think, positive, but it is a hard question. (One argument for a positive answer comes from the hylomorphic view of the human soul—our metabolic life is energized by our rational soul.)