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Each of the following in one way or another questioned the moral values of his day:
Diogenes the Cynic (4th century BC)
While Diogenes had his own clear sense of right and wrong, his values stood in stark contrast with those of Greek society around him. He made it a point to scorn social values he considered obstacles to happiness. According to legend, he once urinated on another person and defecated in the theater. These sorts of antics won him the title, "the dog" (kynē). The "Cynics" thus trace their origins largely to him, a group that advocated a simple life without possessions and considered most of society's rules unnatural.
Pyrrho the Skeptic (ca. 360-270BC)
Pyrrho is the putative originator of the Skeptics in ancient philosophy. The fundamental principle of his philosophy was that we cannot know anything for certain. Equal arguments, he said, can always be made for opposing sides in an argument. This sort of "non-cognitivism," when applied to ethics, leads to moral skepticism--we cannot know whether moral claims are true or false.
Nicolò Machiavelli (1469-1527)
Machiavelli probably held more moral values than one might think from most famous work The Prince, dedicated to a Medici prince. This book is known for the way in which it advocates a political realism that went in the face of the political idealism that preceded him. In The Prince, Machiavelli suggests that a new prince may have to act immorally, using his power and deception to secure his position. In other words, "might makes right" and "history is written by the winners," two sayings that do not come from Machiavelli, but aptly summarize some of his advice.
David Hume (1711-76)
Hume largely believed that our passions and emotions were what stood behind our moral statements. He did not believe that reason had anything to do with our morality, that "facts" and "values" were completely different kinds of things. As such, he is a significant precursor to what would become "emotivism," the idea that moral statements are really statements of emotion. "You should not kill me" really means "I don't want you to kill me" or "I fear you killing me."
Friedrich Nietzsche (1844-1900)
Nietzsche was a moral nihilist. He did not believe that evil truly existed. It was rather an idea that slaves came up with to keep their masters from exerting over them the power that in reality was truly theirs. The masses may need some sense of morality, but the "Übermensch," the "super-person," would always create their own right and wrong.
Michel Foucault (1926-1984)
Although Foucault was notoriously resistant to any label, his sense of morality is largely one that changes over time. For him, power and knowledge were intimately connected. The power structures of a particular time and place create what is true for that time and place. He thus largely played out Nietzsche's perspective in the examination of the history of things like madness, crime/punishment, and sexuality. One must always remember that his examinations of these shifts itself is in his view an exercise of power rather than a search for what was historically true.
J. L. Mackie (1917-1981)
Mackie is perhaps the most notorious moral skeptic/nihilist of recent times. He famously stated that "there are no objective values" and thus that ethics is something we invent rather than discover.