Rights presuppose justice. When I say that I have a right to something, it means that something is owed or due to me from others for some reason. Justice, like

Rights presuppose justice. When I say that I have a right to something, it means that something is owed or due to me from others for some reason. Justice, like all of morality (see "natural law theory"), presupposes human nature. Human beings are rational animals with the freedom to choose according to (or against) what is rational. If we lacked, by nature, either rationality or free choice, it would not make any sense to speak of the exercise of justice and therefore of rights because if I can't understand, then I cannot be expected to choose according to the objectively good, and if I cannot choose freely then I cannot opt for the good. What is good for human beings is objectively true as determined by human nature. It is objectively harmful, for example, to starve. It is objectively harmful to cut off one's right arm. It is also objectively harmful to take drugs that frustrate the exercise of reason because this is opposed to being the kind of thing you are, a human being. The same can be said for the misuse of the body and its faculties in various ways. They work against their healthy function and your well-being. Now, by nature, as I already said, human beings are rational animals, that is, it is our nature to be rational. We are also social animals. Thus, our own flourishing as individual human beings is also social. A society in which justice isn't practiced is no good for the human beings that are a part of it. Justice means that we can make claims, at least under certain conditions. It is of course unjust to make claims upon others that are not warranted, so we must determine what exactly constitutes a just and legitimate claim. This presupposes rationality (you cannot have a claim to what is absurd or evil) and relational (some relations are voluntary, others are not, but the nature of the relationship will inform us of our obligations and claims) and conditioned by other factors (a criminal forfeits certain rights by virtue of having committed an injustice). I will agree with you, though, that "fundamental rights" is unclear. If they mean something like what I've described, as something that is determined by human nature, then sure, they're fundamental in the sense that they have an objective ground in human nature. But if they are understood as somehow absolute in the sense that a criminal could go around murdering people and still maintain a claim to his own life, then no.