definición de Moral_nihilism (Wikipedia)
Moral nihilism (also known as ethical nihilism or amoralism) is the meta-ethical view that nothing is intrinsically moral or immoral. For example, a moral nihilist would say that killing someone, for whatever reason, is neither inherently right nor inherently wrong. Moral nihilists consider morality to be constructed, a complex set of rules and recommendations that may give a psychological, social, or economical advantage to its adherents, but is otherwise without universal or even relative truth in any sense. 
Moral nihilism is distinct from moral relativism, which does allow for moral statements to be true or false in a non-objective sense, but does not assign any static truth-values to moral statements, and of course moral universalism, which holds moral statements to be objectively true or false. Insofar as only true statements can be known, moral nihilism implies moral skepticism.
According to Sinnott-Armstrong (2006a), the basic thesis of moral nihilism is that "nothing is morally wrong" (§3.4). There are, however, several forms that this thesis can take (see Sinnott-Armstrong, 2006b, pp. 32–37 and Russ Shafer-Landau, 2003, pp. 8–13). There are two important forms of moral nihilism: Error theory and Expressivism p. 292.
One form of moral nihilism is expressivism. Expressivism denies the principle that our moral judgments try and fail to describe the moral features, because expressivists believe when someone says something is immoral they are not saying it is right or wrong; expressivists are not trying to speak the truth when making moral judgments, they are simply trying to express their feelings. "We are not making an effort to describe the way the world is. We are not trying to report on the moral features possessed by various actions, motives, or policies. Instead, we are venting our emotions, commanding others to act in certain ways, or revealing a plan of action. When we condemn torture, for instance, we are expressing our opposition to it, indicating our disgust at it, publicizing our reluctance to perform it, and strongly encouraging others not to go in for it. We can do all of these things without trying to say anything that is true." p. 293.
This makes expressivism a form of non-cognitivism. Non-cognitivism in ethics is the view that moral statements lack truth-value and do not assert genuine propositions. This involves a rejection of the cognitivist claim, shared by other moral philosophies, that moral statements seek to "describe some feature of the world" (Garner 1967, 219-220). This position on its own is logically compatible with realism about moral values themselves. That is, one could reasonably hold that there are objective moral values but that we cannot know them and that our moral language does not seek to refer to them. This would amount to an endorsement of a type of moral skepticism, rather than nihilism.
Typically, however, the rejection of the cognitivist thesis is combined with the thesis that there are, in fact, no moral facts (van Roojen, 2004). But if moral statements cannot be true, and if one cannot know something that is not true, non-cognitivism implies that moral knowledge is impossible (Garner 1967, 219-220).
Error theory is built by three principles:
Thus we always lapse into error when thinking in moral terms. We are trying to state the truth when we make moral judgments. But since there is no moral truth, all of our moral claims are mistaken. Hence the error. These three principles lead to the conclusion that there is no moral knowledge. Knowledge requires truth. If there is no moral truth, there can be no moral knowledge. Thus moral values are make-believe.
Error theorists combine the cognitivist thesis that moral language consists of truth-apt statements with the nihilist thesis that there are no moral facts. Like moral nihilism itself, however, error theory comes in more than one form: Global falsity and Presupposition failure.
The first, which one might call the global falsity form of error theory, claims that moral beliefs and assertions are false in that they claim that certain moral facts exist that do not exist. J. L. Mackie (1977) argues for this form of moral nihilism. Mackie, for example, argues that moral assertions are only true if there are moral properties that are intrinsically motivating, but there is good reason to believe that there are no such intrinsically motivating properties (see the argument from queerness and motivational internalism).
The second form, which one might call the presupposition failure form of error theory, claims that moral beliefs and assertions are not true because they are neither true nor false. This is not a form of non-cognitivism, since moral assertions are still thought to be truth-apt. Rather, this form of moral nihilism claims that moral beliefs and assertions presuppose the existence of moral facts that do not exist. This is analogous to presupposition failure in cases of non-moral assertions. Take, for example, the claim that the present king of France is bald. Some argue that this claim is truth-apt in that it has the logical form of an assertion, but it is neither true nor false because it presupposes that there is currently a king of France, but there is not. The claim suffers from "presupposition failure." Richard Joyce (2001) argues for this form of moral nihilism under the name "fictionalism."
The philosophy of Niccolò Machiavelli is sometimes presented as a model of moral nihilism, but this is at best ambiguous. His book Il Principe (The Prince) praised many acts of violence and deception, which shocked a European tradition that throughout the Middle Ages had inculcated moral lessons in its political philosophies. Machiavelli does say that the Prince must override traditional moral rules in favor of power-maintaining reasons of State, but he also says, particularly in his other works, that the successful ruler should be guided by Pagan, rather than Christian virtues. Hence, Machiavelli presents an alternative to the ethical theories of his day, rather than an all-out rejection of all morality.
Closer to being an example of moral nihilism is Thrasymachus, as portrayed in Plato's Republic. Thrasymachus argues, for example, that rules of justice are structured to benefit those who are able to dominate political and social institutions. Thrasymachus can, however, be interpreted as offering a revisionary account of justice, rather than a total rejection of morality and normative discourse.
Criticisms of moral nihilism come primarily from moral realists, who argue that there are positive moral truths. Still, criticisms do arise out of the other anti-realist camps (i.e. subjectivists and relativists). Not only that, but each school of moral nihilism has its own criticisms of one another (e.g. the non-cognitivists' critique of error theory for accepting the semantic thesis of moral realism).
Still other detractors deny that the basis of moral objectivity need be metaphysical. The moral naturalist, though a form of moral realist, agrees with the nihilists' critique of metaphysical justifications for right and wrong. Moral naturalists prefer to define "morality" in terms of observables, some even appealing to a science of morality.
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