I think there's a good reason why reason isn't included in the "big three" metaethical systems: deontology (duty, rules), consequentialism (morality is determined by its consequences, utilitarianism, which utilizes pleasure and pain along these lines, being the major form), and virtue ethics (virtue is central to ascertaining happiness as eudaimonia, or a flourishing life, i.e., well-being of the self).
This is because, I think, reason by itself doesn't give us any direction. Arguing for a life that is lived according to what's most rational doesn't give us starting points (premises) with which to reason, nor does it give us values which might be incommensurate with one another in this reasoning process, causing some people to adopt certain ethical stances relative to the values they hold (a manly man will have values that create premises leading to rational conclusions that are vastly different than the average egalitarian softy).
IOW, I don't think it makes sense to say that you can live your life solely according to reason. Reason is the process of crunching out conclusions from premises. Saying you can live solely according to reason is like saying you can live solely according to culture. Well, which culture? Likewise: which set of assumptions or premises and values direct our reasoning?
Totalitarian socialism had a certain rationality to it, after all.
This is because, I think, reason by itself doesn't give us any direction. Arguing for a life that is lived according to what's most rational doesn't give us starting points (premises) with which to reason, nor does it give us values which might be incommensurate with one another in this reasoning process, causing some people to adopt certain ethical stances relative to the values they hold (a manly man will have values that create premises leading to rational conclusions that are vastly different than the average egalitarian softy).
IOW, I don't think it makes sense to say that you can live your life solely according to reason. Reason is the process of crunching out conclusions from premises. Saying you can live solely according to reason is like saying you can live solely according to culture. Well, which culture? Likewise: which set of assumptions or premises and values direct our reasoning?
Totalitarian socialism had a certain rationality to it, after all.