How does being human give rise to rights? To answer this question we need a theory of human nature. Although I despair of being able to offer one, I can point out some basic distinctions that provide useful insights.
Theories of human nature deal with how we define, or what it means to be, "human." In a very crude way, we can say that a scientific approach to human nature involves an empirical investigation of the psychobiological makeup of human beings. A moral or philosophical approach focuses on what it means to be a person, a human being capable of reflective action and subject to the constraints of morality. Although moral theories may be constrained by science, they address different issues.
Those who seek to ground human rights in science usually speak of basic human needs. But any list of needs that can plausibly claim to be empirically established provides an obviously inadequate list of rights: life, food, protection against cruel or inhuman treatment, and perhaps companionship. The problem may lie in contingent shortcomings of our current scientific procedures or knowledge. I would argue, however, that science is in principle incapable of providing the appropriate kind of theory of human nature. We have human rights not to what we need for health but to what we need for a life of dignity.
An anthropological approach that seeks to ground human rights on cross-cultural consensus faces equally serious problems. History is replete with societies based on hierarchies of birth, gender, wealth, or power. Likewise, many cultures have sanctioned slavery, infanticide, blood feuds, and the execution of dissidents. American history is marked by systematic torture and execution of religious deviants (witches); enslavement of and then legal discrimination against African Americans; barbarous treatment of native peoples; denial of political participation, property rights, and even legal personality to women; and repression of political dissidents (especially communists).
The human nature that is the source of human rights rests on a moral account of human possibility. It indicates what human beings might become, not what they have been historically or "are" in some scientifically determinable sense. Human rights rest on an account of a life of dignity to which human beings are "by nature" suited. If the rights specified by the underlying theory of human nature are implemented and enforced, they should help to bring into being the envisioned type of person, one who is worthy of such a life. The effective implementation of human rights thus resembles a self-fulfilling moral prophecy.
Unfortunately, no philosophical theory of human nature has widespread acceptance. Although consensus is no measure of truth, without consensus any particular theory -- and any action based on it -- is vulnerable to attack. The problem is even more severe when we recognize that many moral theories, and their underlying theories of human nature, deny human rights.
For example, Marxism explains moral beliefs in terms of class structure and struggle, which are determined by the means and mode of production. Radical behaviorists see human personality as the result of conditioning. In both cases, "human nature" is the result of historical processes that shape human beings into socially prescribed molds, rather than the reflection of an inherent essence or potential. For adherents of either theory, talk of equal and inalienable rights held by all people simply because they are human is pointless. "Simply because they are human" probably makes no sense and certainly has no substantive moral implications.
Utilitarianism, which achieved its classic formulations in the works of Jeremy Bentham and John Stuart Mill in the first half of the nineteenth century, is also at odds with human rights. Utilitarians hold that the moral quality of an act is a function of its good or bad consequences (utility). Good and bad, in turn, are matters of pleasure and pain (which are usually understood in subtle and expansive terms). The principle of utility, or what Bentham called the greatest happiness principle, requires us to act so as to maximize the balance of pleasure over pain. For a utilitarian, statements about human rights are at most a convenient shorthand for noting the tendency of certain acts to produce pleasure or pain. Human rights have no independent moral status or force.
Moral or political theories that emphasize differences between communities are also likely to be incompatible with the idea of human rights. Classical Greeks considered themselves inherently superior to "barbarians" (non-Greeks), who were not entitled to the same treatment as Greeks. The American notion of manifest destiny or the British colonial ideology of the white man's burden justified barbarous treatment of nonwhite peoples on the grounds of the superior virtue or moral development of Americans or Englishmen. Nazi Germany provides an even more extreme version of the denial of rights to "inferior races" on grounds of moral and political superiority. More recently, ethnic cleansing in the former Yugoslavia has been based on a claim of unbridgeable qualitative differences between groups. In Israel today, some Zionists and settler communities argue for unbridgeable cultural differences that require rigid physical and political separation and the subordination of Palestinian rights to the apparently "higher" demands of Jewish security.
But there are also a variety of bases for justifying human rights. Human rights have often been held to be given by God. Alan Gewirth has argued that we have human rights to those things that are necessary in order to act as a moral agent. In my own work, I have tried to give an account of human rights as the social and political guarantees necessary to protect individuals from the standard threats to human dignity posed by the modern state and modern markets.
Human rights might also be seen as a political specification of Immanuel Kant's categorical imperative. In his Grounding for the Metaphysics of Morals, Kant argued that there is one supreme principle of morality, namely, the duty to treat people as ends, never as means only. This duty (or imperative), Kant argued, is categorical, without exception. A list of human rights can be seen as a political specification of what it means to treat all human beings as ends.
We thus have a considerable variety of possible moral justifications, as well as an array of theories that deny or radically devalue human rights. In what follows I will assume that there are human rights, that is, that we have accepted some sort of philosophical defense. This theoretical evasion is justified by the fact that almost all states acknowledge the existence of human rights. In other words, this assumption is relatively unproblematic for our purposes here, namely, studying the international politics of human rights.