Many philosophers have thought that the capacity for consciousness or sentience provides a criterion for moral status. Sometimes they distinguish between consciousness and sentience by defining the latter as "the capacity to feel pleasure or pain" (Warren 56). I prefer to use the term 'sentience' in a broader sense to refer two kinds states of consciousness or awareness, sensory awareness and self-awareness. Sensory awareness refers to an organism's capacity to create mental representations of features of its environment, as for instance when it sees, hears, smells, and so forth. Pleasure and pain are conscious states of self-awareness, as are emotions and feelings, for instance, the feeling of hunger. In these kinds of states an organism is conscious of some feature of itself. Psychological organisms that are capable of experiencing pleasure and pain, sensing things in their environments, feeling hungry, and having other states of conscious awareness, have moral standing as things with intrinsic value because they are alive. But being a psychological organism, one capable of conscious states of sensory and self-awareness, gives that entity a different moral standing than living beings who are not capable of consciousness.
Consciousness or sentience is one of the great mysteries of the cosmos. At our present stage of scientific understanding we really do not know what it is or how it arises. Unlike an organism's physical and biological characteristics, we cannot directly observe its consciousness, but can only infer that an entity is (or may be conscious) from external signs. Warren notes that there are four sorts of evidence that we commonly rely upon to determine whether or not something is sentient: 1) whether it has a nervous system, 2) whether or not it exhibits pain behavior such as crying, howling, squirming, or trying to escape when exposed to painful or noxious stimuli, 3) whether it has sensory organs (my daughter's test), and 4) whether its brain and nervous system contain neurochemical transmitters such as those found in human beings and which are associated with the experiencing of states of self-awareness such as pleasure, pain, and various kinds of emotions.
One of the reasons I prefer to talk about psychological organisms rather than consciousness or sentience is that we can determine, using observable criteria such as these, whether or not something should be classified as a psychological organism. Using these observable criteria we can quite readily conclude, for instance, that higher vertebrate animals with brains and central nervous systems like ours, such as are found in mammals and birds, are psychological organisms. Even though other vertebrates cannot report on their own states of consciousness the way human beings can, given their behavioral and physiological similarities to us, we can be reasonably confident that they are also psychological organisms who experience inner states of awareness. My pet parrot, Pierre, must, I think, get pleasurable sensations when I rub his neck.
It gets trickier with "lower" vertebrates such as fish and reptiles whose nervous systems are rather different than ours, and even trickier the farther down we go in the animal kingdom towards invertebrate animals, like worms and insects, oysters and molluscs, and down to unicellular animals such as amoeba. It is a matter of some dispute where exactly we "draw the line" between organisms and psychological organisms, but most people assume that plants, fungi, bacteria, and viruses (if they are even alive) are non-psychological organisms, that is, they are alive but not sentient. But this question need not worry us too much; for most of the kinds of cases where it matters, morally speaking, whether we classify an organism as sentient or not, we will be dealing with creatures that are undoubted psychological organisms.
But why does something being a psychological organism matter to its moral status? It matters, I think, because only psychological organisms can be said to have interests. The avoidance of pain, for instance, is an interest that psychological organisms have. They may also have other kinds of interests, for example, an interest in nourishment, an interest in freedom, or an interest in sex. Having interests is not the same thing as having needs. A plant may need water and sunlight and soil in order to survive and grow, but plants do not have interests. Having interests or desires means that it matters to the organism itself whether its needs are satisfied. Kant used the terms "in itself" and "for itself" to express the idea that human beings have this kind of intrinsic value, a value in themselves and for themselves.
Avoiding pain is valuable for a psychological organism because it can experience pain, and pain is unpleasant for them. Warren puts the point this way: "Non-sentient organisms may have needs, and thus a good of their own, but this is not an experiential good; they experience nothing unpleasant when their needs are thwarted, nothing enjoyable when their needs are met. Consequently, they cannot 'mind' what happens to them, in the ways that sentient beings can" (67).
Moral agents can have moral responsibilities towards non-psychological organisms, such as plants and bacteria, but I do not think it makes sense to say that non-psychological organisms can have rights. In order for something to be said to have a right it must be the case that it is the sort of thing that can have interests, and those interests can be affected for good or ill by the actions of moral agents. Thus, while I think that, say, a chimpanzee, is the sort of thing which can be the holder of rights, the tree in my front yard is not. Put another way, not all moral patients qualify as possible right-holders; psychological organisms do, but non-psychological organisms do not. In saying this I am not claiming that chimpanzees have any rights, only that they could have them, that it would be meaningful to ascribe rights to them because of the kind of thing that they are. Other sorts of things, for instance, inanimate objects such as pebbles and chairs cannot have rights; it makes no sense whatsoever to talk about the rights of pebbles. It does, however, make sense to talk about some kiind of corporate entities, such as corporations, as having rights. Perhaps, as some philosophers and others have suggested, one day we will need to extend the concepts interests and rights to cover certain kinds of artificially intelligent robots. I leave these questions to one side, since for the time being I am only interested in the kinds of moral status that pertain to life forms.
Joseph Raz proposed the following definition of a right: "'x has a right' if and only if x can have rights, and other things being equal, an aspect of x's well-being (his interest) is a sufficient reason for holding some other person(s) to be under a duty" ["On the Nature of Rights" Mind, 93 (1984), 195]. Plants, bacteria and other non-psychological organisms, on my view, have a "well-being" but since they do not have interests cannot be the sorts of things that can have rights. Psychological organisms can have rights, and when they do, there is some interest they have that provides a sufficient reason for moral agents to have a responsibility towards them. Having rights, however, does not mean that it is never morally permissible to treat them in ways that would seem, on first face (prima facie) as contrary to their interests or violative of their rights. Not all interests are protected by rights, and rights normally carry with them various kinds of defeasibility conditions that permit certain kinds of exceptions and limitations. This can be made clearer by means of some examples.
Many animal rights advocates believe that it is morally wrong to inflict pain and suffering on sentient animals in order to satisfy trivial human interests. They argue, for instance, that we should not subject rabbits to painful experiments simply in order to test cosmetics. We have laws now forbidding various kinds of cruelty to sentient animals; people can be punished judicially, for instance, for staging dog fights or cock fights. On the other hand, many people also think that it is morally permissible to use sentient animals in medical experiments whose goal is to find safe and effective treatments for serious human diseases.
As Warren argues, a multi-level theory of moral status that treats sentient animals differently than non-sentient organisms, and persons as different than sentient animals, does a better job of accounting for the moral intuitions that underlie these kinds of moral judgments since it entails that "sentient beings should not be subjected to pain and suffering, except in the service of needs that are important and cannot otherwise be served" (86). Enjoying the spectacle of dogs tearing each other to pieces is not a good enough reason to permit such activities no matter how many people enjoy it, because sentient animals have rights not to be subjected to needless cruelty. Versions of utilitarianism, such as preference utilitarianism, that aggregate pleasures and pains across all sentient beings, do not yield the same conclusion, for they allow that the aggregate pleasure of the human spectators to dog fights might out-weigh the pain it causes to the animals involved. This is one reason why having "rights" matters morally, and why I think it makes sense to say that psychological organisms are the kinds of things that can have rights, that is, they form a class of moral patients who occupy a moral plateau on which it is possible to ascribe rights to them.
Whether we decide to ascribe certain rights to certain psychological organisms is a question that will have to be settled by means of the usual processes by which we socially legitimize rights and right ascriptions. Having a right means that the sort of moral responsibilities or duties that moral agents have towards the right-holder are peremptory, that is, we can demand that they fulfil these duties, and we can enforce that demand by means of the coercive police powers of states. So, if people should be judicially sanctioned for cruelty to animals for failing to fulfill a moral duty they have towards them, as I think they should, then a precondition for doing so is that the moral patients who are the objects of their duties must be the kind of things that can have rights.
However, people should not be judicially sanctioned for killing flies, stepping on spiders, using weed-killer or anti-biotics, or for various other kinds of human activities that harm non-psychological organisms. In these cases, my way of thinking about moral status entails that while moral agents can have prima facie moral responsibilities not to needlessly harm them, they cannot be required to fulfill these kinds of moral responsibilities, at least not by means of coercive sanctions, because non-psychological organisms are not the kinds of things that can have rights, and they cannot have rights because they are not the kinds of things that have interests.
The capacity for having interests, being sentient or qualifying as a psychological organism, then, grounds a particular type of moral standing that is different than the type that applies to non-sentient organisms. However, it is necessary to complicate this theory. Warren observes that "it is implausible to suppose that all sentient beings can be fitted into just two categories of moral status, one for persons and another for all the rest" (87). It is implausible because sentience or consciousness is not an all-or-nothing affair like being alive or not alive; there are degrees of sentience or consciousness. She proposes that we think about a "sliding scale" of sentience in our judgments about moral status because,
There is no obvious place on the phylogenetic scale to draw a line between self-aware beings and those that are not self-aware; or between minimally sentient organisms and those that are wholly non-sentient. A sliding scale of moral status enables us to avoid the distasteful task of sorting animals into those that have first-class status, those that have second-class status, and those that have no moral status at all. It also reduces the need to determine the precise location of the sentience line, since on a sliding scale the moral status of minimally sentient beings may be only slightly different from that of non-sentient organisms (87).This is one reason why I prefer to use the term moral stature when speaking about gradations of moral standing. All organisms have the moral status of ends-in-themselves, that is, as entitites having a kind of intrinsic moral value. This means that they have moral standing and can therefore function as the objects of the moral responsibilities of moral agents. Undoubted psychological organisms have a different level of moral standing than non-sentient organisms because they have interests and consequently can have rights.
But psychological organisms also have different degrees of moral stature, depending on how developed and complex their psychological capacities are. The greater an organism's psychological capacities are, the higher its moral stature, the weightier the reasons must be to excuse moral agents from their moral responsibilities towards them. The moral stature of moral patients can also be increased or decreased by means of the relational criteria for assigning moral status that Warren proposes. So, for instance, a beloved family pet would have a greater moral stature than a similar organism who is not a member of an interspecific moral community but has a similar set of psychological capacities.
But even among psychological organisms considered in themselves, there is a hierarchy of moral statures. So, for instance, suppose you are doing a medical experiment to test a drug for possible use against a human disease. Suppose further, that you can use either lab mice or chimpanzees as your animal model, but in either case, you will have to inflict some pain upon, or perhaps even kill, your animal subjects. In such a case, my moral intuition tells me that it is morally preferable to use the mice because, although they are probably sentient, their psychological capacities, and the form of consciousness they support, is less complex than that of the chimpanzees. Chimps have greater moral stature than mice although they both have moral standing as psychological organisms. An entity's moral status, then, can be seen as a function of both its level of moral standing and its moral stature on that level. Chimps occupy a higher moral plateau than do mice, other things being equal, because their more complex psychological capacities give them higher moral stature.
This way of talking, this "language game", will have other uses. The concepts of moral standing, moral stature, and moral plateaus, will be useful later on when we consider various questions about our moral responsibilities towards other human beings. In particular, when we consider human beings from an ontogenetic perspective, that is, from the point of view of morphogenesis, I will want to argue that human embryos have a different kind of moral standing than sentient human fetuses, which have a different kind of moral standing than competent adult moral agents, or persons. I will also want to argue that human individuals grow in moral stature as they negotiate the passage between embryohood and adulthood. Moral agents, on this kind of view, have moral responsibilities to protect vulnerable human individuals at all stages of morphogenesis, but the content and strength of those moral responsibilities change as the individual develops its psychological capacities and grows in moral stature.