Some defenders of moral skepticism include Pyrrho, Aenesidemus, Sextus Empiricus, David Hume, J. L. Mackie (1977), Friedrich Nietzsche, Richard Joyce (2001), Joshua Greene, Richard Garner, Walter Sinnott-Armstrong (2006b), and the philosopher James Flynn.
The most famous moral error theorist is J. L. Mackie, who defended the metaethical view in Ethics: Inventing Right and Wrong (1977).
The first argument people attribute to Mackie, often called the argument from queerness,[2] holds that moral claims imply motivation internalism (the doctrine that "It is necessary and a priori that any agent who judges that one of his available actions is morally obligatory will have some (defeasible) motivation to perform that action"[3]).
The other argument often attributed to Mackie, often called the argument from disagreement,[3] maintains that any moral claim (e.g. "Killing babies is wrong") entails a correspondent "reasons claim" ("one has reason not to kill babies").
This includes the psychopath who takes great pleasure from killing babies, and is utterly miserable when he does not have their blood on his hands.
For example, Michael Ruse[4] gives what Richard Joyce[3] calls an "evolutionary argument" for the conclusion that we are unjustified in believing any moral proposition.
He highlights the persistent debates on whether the basis of right action is rooted in reasons or consequences, and the diverse, conflicting theories within Western moral philosophy.
Thus the argument from the non-instantiation of (what Mackie terms) "objective prescriptivity" for moral error theory fails.