This is the opening post in a series on the challenge of marginal cases and several prominent responses to that challenge.
Most people believe that human beings have full moral significance, whereas nonhuman animals have only partial moral significance. For example, most people believe that the use of nonhuman animals in vivisection may be morally justified, provided the practice yields clear and significant benefits and provided the animal subjects do not suffer ‘gratuitously.’ Most people would also claim that morality categorically prohibits the use of human beings in vivisection, no matter how clear and significant the benefits or how ‘humanely’ the human subjects were treated. Let’s call this view about the unequal moral significance of human beings and nonhuman animals the conventional view.
Accompanying the conventional view is a conventional justification–viz., human beings and nonhuman animals have unequal moral significance because human beings have capacities that nonhuman animals lack. For example, human beings, unlike nonhuman animals, are capable of understanding morality and governing their conduct in accordance with its norms. The form of justification here pays tribute to an intuitively compelling higher-order moral principle, which we may call the relevant difference principle, and state as follows: If two individuals, A and B, have unequal moral significance, there must be a morally relevant difference between A and B.
Suppose we assume that nonhuman animals lack the capacity for moral agency, and that lacking that capacity is what justifies our treatment of them, or what explains why they have less than full moral significance. There are human beings who lack this capacity as well. Indeed, there are human beings whose cognitive and emotional endowment is roughly equivalent, and in many cases inferior, to that of certain nonhuman animals. Just to fix ideas, picture a healthy adult chimpanzee alongside a profoundly mentally retarded human being. What should we say about the moral significance of the severely disabled human being?
Perhaps one is prepared to say: ‘Ah, yes–there are human beings, too, who have only partial moral significance. Other things equal, we are as morally justified in using them in vivisection as we would be their nonhuman animal counterparts. That we don’t in fact use them in this way has more to do with extra- or non-moral considerations than moral ones.’
Most people, I suspect, are not prepared to say this: the profoundly mentally retarded human being has full moral significance. Enter the challenge of marginal cases, which confronts us with what seems like impeccable reasoning:
1. Relevant difference principle: If two individuals, A and B, have unequal moral significance, there must be a morally relevant difference between A and B.
2. There is no morally relevant difference between (A) a profoundly mentally retarded human being and (B) a nonhuman animal whose cognitive and emotional endowment is roughly equivalent to (A)’s.
3. (A) has full moral significance.
4. Therefore, (B) has full moral significance.
The relevant differences principle seems to capture something essential to moral reasoning; it appears pretty invulnerable. Since we’re assuming the truth of (3), that leaves (2) as the only viable target. We either have to explain why (2) is false, or, assuming the argument’s validity, give up the conventional view.
Not surprisingly, what we find in the literature on the challenge of marginal cases are attempts to refute (2)–that is, attempts to identify a morally relevant difference between (A) and (B). In the next post I’ll review one such attempt and explain why I think it fails.