Why bother with moral philosophy when common sense serves most of us perfectly well? The simple answer is that, as history shows, commonsensical beliefs are very often wrong. Slavery, marital rape, and bans on interracial marriage were all widely accepted in the relatively recent past. Much like fish who, as the proverb goes, are the last to discover water, humans are so immersed in immorality that we can be entirely unaware of it.