It is the fourth criticism that, he says, "is lethal to the whole idea of a natural or human right" (p. 146). So what is it? I'm not sure, but I'll do my best to make it out. It "is directed at the nature of a 'natural right'." (p. 143) Geuss says that he has "strongly suggested" (he doesn't say where and I don't know, but then I haven't read the whole book, so that's likely to be my fault) that it is "essential to the existence of a set of 'rights' that there be some specifiable and more or less effective mechanism for enforcing them" (p. 143). Presumably this suggestion is based on some kind of conceptual analysis, but Adam Smith's idea of moral rights as rights in a metaphorical sense, and J. S. Mill's idea of rights as "essentials of well-being," suggest a different conception of rights than the one Geuss seems to have in mind. So I don't accept his suggestion.
On p. 144 he writes:
Either there is or there is not a mechanism for enforcing human rights. If there is not, it would seem that calling them 'rights' simply means that we think it would (morally) be a good idea if they were enforced, although, of course, they are not.He doesn't say why it would seem that this is the case. I don't see why someone couldn't think that something was essential for living a good human life, or flourishing, and yet not want to see force used to ensure that everyone gets this something. Perhaps the violence necessary for enforcement would be worse than the violation of the right. But I don't mean to exaggerate my disagreement with Geuss here.
He then discusses the possibility that declarations of rights might somehow become enforced, and says:
The question is not whether this is possible or whether it would be a good thing, but whether such a development is the invention of a new set of positive 'rights' in a new international legal system or the emergence into visibility of a set of natural human rights that already existed. (p. 145)He sees the point of appeal to natural or human rights as being undone if we admit they are "something we made to exist" (p. 145). They are, after all, supposed to be "something we discovered which served as the grounds for judging actual legal rights" (p. 145). This does not fit the idea of moral rights as the imperfect cousin of legal rights, but it does fit one kind of idea of human rights. So let's stick with that conception of rights for now. What could provide such grounds? "The only thing that can serve that purpose seems to be the flickering light of our variable moral beliefs" (p. 145 still). Huh?
Consider some examples: the right to freedom of speech, the right to equal opportunity regardless of race or gender, the right to freedom of religion. To say that such rights exist, according to Geuss, is to say that it would be morally good if the law were used to protect such freedoms and opportunities. Did we make this moral goodness exist? I don't think so. Did we discover it to exist? Perhaps not literally, but yes, I think we did. (Or we realized that it existed, or came to accept its existence, or something like that.) Sometimes our moral beliefs vary because we come to appreciate something more or better than we had before. Why not call this discovery? What else should we call our having come to regard the sexes and races as being morally equal, as requiring (as a matter of right) equal treatment and opportunity? Flickering? Waffling? I don't buy it.
In the end Geuss seems to think that belief in human rights is "only a moral belief" (p. 146). He then adds that people's moral beliefs vary, and that agreement does not guarantee effective action. He concludes that there "are no natural rights" (p. 146). I think this is because of the lack of the enforcement mechanism he thinks is essential to the very idea of a right. If we give up that requirement then his argument seems to be reduced to the observation that a belief in human rights is a moral belief. And I agree with that, but don't see it as a problem.