Gewirth has written a great deal about human rights - moral rights that human beings have because they are human. Broadly speaking, the rights he argues for are the rights to freedom and well-being. He believes that he can demonstrate that individuals have these rights because they are the necessary conditions for all action. Here is a summary of his argument in his own words:

First, every agent holds that the purposes for which he acts are good on whatever criterion (not necessary a moral one) enters into his purposes. Second, every actual or prospective agent logically must therefore hold or accept that freedom and well-being are necessary goods for him because they are the necessary conditions of his acting for any of his purposes; hence, he holds that he must have them. Third, he logically must therefore hold or accept that he has rights to freedom and well-being; for, if he were to deny this, he would have to accept that other persons may remove or interfere with his freedom and well-being, so that he may not have them; but this would contradict his belief that he must have them. Fourth, the sufficient reason on the basis of which each agent must claim these rights is that he is a prospective purposive agent, so that he logically must accept the conclusion that all prospective purposive agents, equally and as such, have rights to freedom and well-being. [Gewirth, Human Rights: Essays on Justification and Applications, p. 20]

Gewirth has elaborated this argument in great detail in his book Reason and Morality. He is one of the few contemporary philosophers who attempts a proof of moral principles in a very strong sense of the word 'proof.' If he is correct, ethical relativism is mistaken about the nature of fundamental ethical principles.