From The Art and Popular Culture Encyclopedia
"Moral skepticism" denotes a class of metaethical theories all members of which entail that no one has any moral knowledge. Many moral skeptics also make the stronger, modal, claim that moral knowledge is impossible. Moral skepticism is particularly opposed to moral realism: the view that there are knowable, mind-independent moral truths.
Defenders of some form of moral skepticism include J. L. Mackie (1977), Max Stirner, Friedrich Nietzsche, Richard Joyce (2001), Michael Ruse, Joshua Greene, Richard Garner, Walter Sinnott-Armstrong (2006b), and the psychologist James Flynn. Strictly speaking, Gilbert Harman (1975) argues in favor of a kind of moral relativism, not moral skepticism. However, he has influenced some contemporary moral skeptics.