This is a philosophical exploration of the concept of moral status. To have moral status is to be morally considerable, or to have moral standing. It is to be an entity towards which moral agents have, or can have, moral obligations. (p. 3)
I think this opening passage exhibits a flaw in the whole notion of moral status, but I will get to that in a moment. After some elaborate and interesting discussion of single-criterion accounts of moral status, Warren goes on to propose her own multi-criterial analysis. In this analysis, moral status is pinned down using seven principles, all of which are supposed to be "implicit elements of common-sense morality" (p. 149). They are as follows:
1. Respect for Life Principle: Living organisms are not to be killed or otherwise harmed, without good reasons that do not violate principles 2-7.
2. Anti-Cruelty Principle: Sentient beings are not to be killed or subjected to pain or suffering, unless there is no other feasible way of furthering goals that are (1) consistent with principles 3-7; and (2) important to human beings, or other entities that have a stronger moral status than can be based on sentience alone.
3. Agent's Rights Principle: Moral agents have full and equal basic moral rights, including the rights to life and liberty.
4. Human Rights Principle: Within the limits of their own capacities and of principle 3, human beings who are capable of sentience but not of moral agency have the same moral rights as do moral agents.
5. Ecological Principle: Living things that are not moral agents, but that are important to the ecosystems of which they are part, have, within the limits of principles 1-4, a stronger moral status than could b ebased upon their intrinsic properties alone; ecologically important entities that are not themselves alive, such as species and habitats, may also legitimately be accorded a stronger moral status than their intrinsic properties would indicate.
6. Interspecific Principle: Within the limits fo principles 1-5, non-human members of mixed social communities have a stronger moral status than could be based upon their intrinsic properties alone.
7. Transitivity of Respect Principle: Within the limits of principles 1-6, and to the extent that is feasible and morally permissible, moral agents should respect one another's attributions of moral status.
I think there are a number of problems with the whole notion of moral status when it is given any serious moral weight. One of the reasons for this is that the apportionment of moral status is itself a morally risky thing: it is, at its most basic level, the division of the universe into castes, and then it proceeds to give us, as falling in the caste of moral agents, a list of occasions on which we can deny that the other castes have any relevant moral standing. Human beings have a long history of doing precisely this, actually; and it has often not been a good thing. To be sure, that we've often drawn the caste-lines on the basis of prejudice does not of itself imply that we always will. But we would need some excellent assurance that prejudices are not interfering with the drawing of accurate lines. This is particularly true given that accounts of moral status always seem somewhat gerrymandered. Single-criterion accounts manage to minimize this appearance by sticking to a single principle; but they run into a number of problems that Warren discusses in some detail. Multi-criterial accounts like Warren's can avoid these problems; but the danger of gerrymandering increases exponentially, and the more principles you have, the more ad hoc it seems to have these and only these principles. (6) above, for instance, is clearly thrown in chiefly to increase the moral status of pets; (4) is probably not the formulation that most people would consider natural, but it becomes clear enough that it has been carefully and deliberately balanced in order to give the right to life to patients in a temporary coma and to infants while not restricting abortion; one might well argue that these principles are crucially incomplete; and so forth. The principles are supposed to capture implicit elements of common-sense morality; but both the common sense and the explication of its implicit elements is easily overrun by personal opinion. Without having a solid reason to think that this has not happened, the whole exercise is largely pointless.
Moral status is, in any case, an otiose concept. Warren throughout her discussion -- and in this I think she is not alone -- treats moral status ambiguously. This arises in the first few sentences. On the one hand, to have moral status is to be morally considerable, to have moral standing. On the other, it is to be something to which one can have obligations. There seems to be a tendency to conflate the two, when there are good reasons for not doing so. Early in the book Warren takes an example of a stone to try to give an intuitive motivation to the notion of moral status:
Is it morally wrong to take a stone and grind it into powder, merely for one's own amusement? Most people would say that it is not--unless there are special circumstances. Perhaps the stone belongs to someone for whom it carries precious personal memories. Perhaps it contains fossilized dinosaur bones from which important scientific knowledge could be gleaned, or valuable gems which could be sold to feed starving people. In these cases, we might say that it would be wrong to destroy the stone for no good reason. But most of us would regard it as a wrong only in so far as it causes harm to human beings, or deprives them of important benefits. The stone itself does not seem to be the kind fo thing towards which we can have moral obligations. (p.4)
This captures very well the difference between the two. We have no obligations to stones. But it does not follow from this that stone is not morally considerable; Warren here gives a number of reasons why the stone might well be morally considerable. To have moral standing usually means that it makes a moral difference how one is treated; whether the stone has moral standing may depend on the context, but sometimes it will.
Warren notes that the notion of moral status has two functions: to establish minimal standards and to establish moral ideals. These are radically different functions, and cannot be fulfilled simultaneously. (Warren herself notes this, arguing that it can be problematic to propound moral ideals as minimal standards.) The natural conclusion is that 'moral status' is equivocal and really just lumps together different kinds of moral reasoning that are more fruitfully kept distinct.
When we recognize this, it becomes very clear that what the notion of moral status clumsily captures is this: the place something occupies in good moral reasoning. To the extent that the criteria for moral status have any value or plausibility, it is because they capture patterns of moral reasoning that are fairly common. But these patterns can be examined, evaluated, and applied in their own right; there is no reason whatsoever to try to capture them by such a roundabout and obscure means as the notion of moral status.
None of this is to say that 'moral status' might not be a perfectly reasonable term to use in some colloquial contexts. But it contributes nothing of significance to moral reasoning, being nothing more than a clumsy ossification of moral reasoning itself.
Quotations are from Mary Anne Warren, Moral Status, Clarendon (Oxford: 1997).