The Universal declaration of 1948 celebrated the belief in human rights as a great moral value. But what does this belief mean exactly? What are human rights precisely? Admitting the existence of human rights may cause difficulties for the moral theories involved and raise many problems. The problem of justification is particularly relevant: are human rights grounded on nature, that is on something unalterable and absolute, or are they the product of history and social life? Different moral theories of human rights give different answers. This paper, therefore, tries to investigate the controversial question of the justification of human rights by comparing the two main positions forwarded in proof of their existence, naturalization and denaturalization, which are developed inside the main moral theories of human rights. After showing the advantages and disadvantages of these rival arguments, some conclusions are drawn that could throw some light on the question of the justification of a concept, such as that of human rights, on which our present social life appears to be intrinsically based.