Do not lie...except when....

Let’s consider a moral rule like “Do not lie.” 

this can can be expanded into “Do not lie unless it is to save an innocent life.” Kant actually failed to understand that exceptions could be built into his categorical imperatives, which is why he believed that lying was morally wrong no matter what (even if it was lying to save someone’s life, such as a Nazi asking about Jews hiding in your attic). 

Prior to understanding that exceptions can be built into rules to accommodate situational ethics, the rules work a lot less well (even though they still work better than no rules at any time). But once you add these exceptions in, you can make the moral framework a lot more useful and still easy enough to follow without too much mental effort. 

One can in fact accommodate any number of psychological idiosyncrasies into these exceptions as well (don’t do this unless you have such and such a brain condition, upbringing, etc.), but obviously doing this makes the set of rules much more complex than they otherwise would be, and therefore harder to memorize and recall on short notice.