Dear philosophers, I had two queries about Kantianism, and was wondering if anyone could assist. There's a letter of Kant's in which he says, essentially, that if a murderer comes to your door asking where your friend is, you may not lie to him, because the principle of allowing lies is not something that can be consistently maximised. I was wondering: (1) is there a problem of how to categorise an action? I mean, is the principle here, "It's OK to lie" or is it "One should not assist murderers"? How do you definitively characterise an action? (2) is there a problem of complexity of maxim? If one agrees that "It's OK to lie" can't be maximised, what about if exceptions are built in? "It's OK to lie to murderers who are likely to believe your lie" -- could something like that be maximised?