The argument from marginal cases is an argument that attempts to show that if we have moral duties to “marginal cases,” by which is generally meant infants, the senile, the mentally disabled, etc., then we also have moral duties to non-human animals. The reasoning is that non-human animals have far greater mental capacities than marginal cases so there is no morally good reason for discriminating against them. It is, one might say, an argument against speciesism.
Here is a summary from the internet encyclopedia of philosophy:
The Argument from Marginal Cases is an argument that attempts to demonstrate that if animals do not have direct moral status, then neither do such human beings as infants, the senile, the severely cognitively disabled, and other such “marginal cases” of humanity. Since we believe that these sorts of human beings do have direct moral status, there must be something wrong with any theory that claims they do not. More formally, the argument is structured as follows:
If we are justified in denying direct moral status to animals then we are justified in denying direct moral status to the marginal cases.
We are not justified in denying direct moral status to the marginal cases.
Therefore we are not justified denying direct moral status to animals.
If as I’ve argued previously rights are determined in contractarian fashion then the second premise in the above argument is false. That would mean that it is the case that we do not have direct obligations to marginal cases. That is of course a bit strange on the face of it.
It means that it is wrong to kill children not because the children have a right to life but because we can’t allow other people to kill children for our own safety. If people start killing their children they are more likely to harm me or my child.
It is a weird view in terms of morality, which is why I myself am not a contractarian in terms of how I understand morality. But again there is a difference between morality and rights.
While it is technically true that children don’t have a right to life, we as a society have evolved to live up to the higher standard of morality on this issue such that we all agree it is wrong to kill children.This is something that we have not done with animals, yet.
It is probably worth noting that all marginal cases are not alike. I think it could be argued with plausibility that one could allow the killing of severely demented and mentally retarded human beings while not allowing the killing of babies. This would be because killing of the severely disabled would probably not have the same psychological effect as the killing of babies would.
Jane English in her paper “Abortion and the Concept of a Person” argues along these same lines. She argues that abortion in the early stages of pregnancy is morally permissible because it doesn’t compromise a persons moral psychology but that abortion in the late stages is too similar to killing a fully developed human being and is therefore not morally permissible.
Contractarianism is a very primitive, or you might say basic, conception of morality, rights, and obligations. Traditional societies were generally contractarian which is why women, children, animals, other tribes, slaves, etc. weren’t given full moral status.
Moral progress in humanity has been a gradual evolution from contratarian ethics to a more universal contractualist or Kantian style ethics. Given enough time we might eventually make the complete transition to a completely universal system of morality.