In meta-ethics, moral skepticism is a theory which maintains either that ethical claims are generally false, or else that we cannot sufficiently justify any ethical claims, and must therefore maintain doubt about whether they are true or false.

For example, the claims "it is wrong to kill" and "it is acceptable to kill" are both false, according to the first version of moral skepticism. The moral skeptic says that this is because ethical claims implicitly pre-suppose the existence of objective values, and that these do not exist.

Such a position is exemplified in J. L. Mackie's book Ethics: Inventing Right and Wrong. Mackie's main argument against the existence of objective values is the argument from queerness - objective values would be very queer things indeed, very different from everything else in the world - indeed, they would have to be something like the Platonic forms (which Mackie considers a "wild product of philosophical fancy"). Furthermore, how we are supposed to discover these objective values is mysterious.

The moral skeptic's conclusion is that objective values are merely useful fictions for the preservation of society.