Excerpt from Christine Korsgaard, Fellow Creatures, Oxford University Press, 2018.
5.2.3
There is a third reason for rejecting the argument from marginal cases, and it is the most important. A creature is not just a collection of properties, but a functional unity, whose parts and systems work together in keeping him alive and healthy in the particular way that is characteristic of his kind. Even if it were correct to characterize a human being with cognitive defects as "lacking reason," which usually it is not, this would not mean that it was appropriate to treat the human being as a non-rational animal.
Rationality is not just a property that you might have or lack without any other difference, like blue eyes. To say that a creature is rational is not just to say that he has "reason" as one of his many properties, but to say something about the way he functions. He thinks about the things he is inclined to believe or to do, assesses them by rational standards, and believes or acts accordingly. A rational being who lacks some of the properties that together make rational functioning possible is not non-rational, but rather defectively rational, and therefore unable to function well. He is unable to make good decisions about what to believe or to do.
Children, for example, gradually acquire the kind of self-consciousness that makes us rational. They are, increasingly as they grow up, aware of the grounds of their beliefs and actions. As we saw before (3.2), it is because of this form of self-consciousness that human beings need reasons in order to believe and act, and principles from which to derive those reasons. A child who has the relevant form of self-consciousness but is not yet able to understand the principles that should guide his choices is unable to make good decisions, and therefore needs the help of adults to decide what to do.10 Having developed one aspect of rationality before the other, he is not yet functionally unified.
An addict, on the other hand, may be able to evaluate her desires correctly, but may be unable to refrain from acting on them. A comatose person does not have a different way of functioning, but rather is unable to function at all.
It is not as if you could simply subtract "rationality" from a human being, and you would be left with something that functions like a non-human animal. A non-rational animal, after all, functions perfectly well without understanding the principles of reason, since he makes his choices in a different way. He is "designed" by the evolutionary process to be guided by the ways he instinctively perceives the world, rather than by reason (3.2).
In fact, this reflection gives me a way to bring out the point I am trying to make. I have been suggesting that the argument from marginal cases wrongly treats a way of functioning as if it were just a property you could have or lack without other changes. In general, it treats a living thing as a heap of properties rather than a functional unity. The picture it suggests is this:
- Properties of a dog: alive, sentient, emotional, intelligent.
- Properties of a human being: alive, sentient, emotional, intelligent, rational.
- Subtract rationality from the human being: you are back to something essentially like the dog.
To see what is wrong with that, you need only consider the following analogy:
- Plant: alive, capable of nutrition and reproduction.
- Dog: alive, capable of nutrition, reproduction, and action.
- Subtract the capacity for action from the dog: you are back to something essentially like the plant.11
No, you are not back to something essentially like the plant. Subtract the capacity for action from the dog, and the dog will die. The most obvious reason for this is that the way the dog carries out the function of nutrition is through action. A dog has to be able to go and get his food, and before he can digest it he must eat it, and those are actions. A dog who was, say, paralyzed, and therefore unable to act, would, without assistance, starve to death.
In the same way, a severely mentally defective human being could not simply revert to functioning completely instinctively. It is not like we could decide: well, he is not quite fit for human society, but if you just let him loose in the forest he will be fine. Many things about the human being, including the way that we act, are changed by the fact that we are rational. In a much more extensive way than the other animals, we have to figure out what to do, and a severely mentally defective person cannot do that. That is why we have to take care of such people. They are rational beings with defects that make them unable to function rationally. If they were not rational beings, they would not have a problem.
The Argument from Marginal Cases ignores the functional unity of creatures. A creature who is constructed to function in part by reasoning but who is still developing or has been damaged is still a rational creature. So the Kantian need not grant and should not grant that infants, the insane, the demented, and so on, are non-rational beings.
The point is not, of course, that we should treat infants and people with cognitive disabilities exactly the way we treat adult rational beings, because they too are rational beings. The way we treat any creature has to be responsive to the creature's actual condition. But the creature's condition itself is not given by a list of properties, but also by the way those properties work together. The moral issues raised by the fact that creatures grow up through developmental stages, the moral issues raised by the fact that all creatures are subject to illness and handicap and damage, and the moral issues raised by the fact that creatures come in different species are very different kinds of moral issues, and should not all be treated alike.12