The power of skepticism depends on the apparent possibility of rationally asking, for virtually any kind of proposition commonly thought to be known, how it is known or what justifies believing it. Moral claims are among those commonly subjected to skeptical challenges and doubts, even on the part of some people who are not skeptical about ordinary claims regarding the external world. There may be even more skepticism about the possibility of justifying moral actions, particularly if they are against the agent's self-interest. Both problems – how to justify moral claims and how to justify moral action – come within the scope of the troubling question ‘Why be moral?’ Even a brief response to moral skepticism should consider both kinds of targets of justification, cognitive and behavioural, and should indicate some important relations between the two types of skeptical challenge. I will begin with the cognitive case – with skepticism about the scope of theoretical reason in ethics – proceed to practical skepticism, which concerns the scope of practical reason, and then show how an adequate account of rationality may enable us to respond to moral skepticism.

I Skepticism: general, moral, and practical

There are many kinds of skepticism, far more than can be even catalogued here. Call the view that there is no knowledge or justification, whether theoretical or practical, comprehensive general skepticism. This has rarely been held, but the view that no one has knowledge or justified belief regarding the external world – a kind of general cognitive skepticism – is an arguably defensible position. (Comprehensive cognitive skepticism – the view that there is no knowledge or justified belief whatever – has not been plausibly defended.) Most who are inclined to hold this version of external world skepticism would also tend to maintain that there is no moral knowledge and no justification for holding moral views. Call that position cognitive moral skepticism.