It is written in the Book of Declarations:
We hold these truths to be self-evident, that all men are created equal, that they are endowed by their Creator with certain unalienable rights, that among these are life, liberty, and the pursuit of happiness. That to secure these rights, governments are instituded among men, deriving their just powers from the consent of the governed, that whenever any form of government becomes destructive of these ends, it is the right of the people to alter or abolish it, and to institute new government, laying its foundation on such principles and organizing its powers in such form, as to them shall seem most likely to effect their safety and happiness.
I (almost) believe that the statement that human individuals have rights to their lives, to their freedom, and to their property, among other things, is a statement which is just as true as the statement that the electrical charge of the proton is approximately 1.602 * 10^-19 coulombs, or the statement that the square root of 2 is not a rational number.
But how can that be true? Isn't it absurd to think of 'human rights' as anything other than a social construct?
While it would, at first sight, seem obvious that this must be the case, it, too, leads to an absurdity, an even worse one. If "human rights" are whatever we may choose to say they are, then what was wrong with the Holocaust, and what was wrong with Negro slavery?
Natural rights theory, therefore, even if it seems to suffer from epistemological problems, is superior to the theory that your fundamental rights are what the government says they are.
Your legal rights, enforceable in the courts of a given country, of course, are what the government has put into law. But how well those conform with actual human rights is the basis which allows us to criticize a law as just or unjust.
Legislation is a social construct, but so is culture. Thus, one could say that human rights are not what any one government legislates, but what has emerged from... authentic folk sources.
It is true that the folk beliefs about fairness of the peasantry are superior to the decree of any one tyrant.
We can even view those beliefs as a root from which true knowledge of human rights can spring, as in the fullness of time additional things not apparent to simple peasants might emerge; the equality of women would be an obvious example, an idea that emerged as advancing civilization reduced the need for a division of labor by sex.
I personally am most fond of this approach to socially constructing a definition of human rights: start from principles known from peasant wisdom, such as defining fairness in terms of symmetry, and then reason can access the abstraction of human rights if it is applied dispassionately and without bias.
And so we would take the ancient Greek philosophers as our model.
Absolute natural rights are assumed to exist in some sense, even if we cannot really put our finger on what that sense might be at this time, and to posess such properties as seem obvious to our basic moral sense, such as fairness, and starting from small beginnings, more can be inferred.