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?

Thanks for your question.

Before my response, a brief observation: You speak of principles being "maximized". I suspect you're confusing Kant's notion of a maxim with some other idea from moral philosophy (perhaps the utilitarian claim that right actions are those that maximize well-being). A maxim, for Kant, is a justifying principle of action with the form "I will do act A in circumstances C to achieve end E". Kant's Formula of Universal Law requires that we act only on maxims that can be consistently universalized. There's a lot of controversy as to precisely what it means for a maxim to be consistently universalizable. Here's a description, provided by Robert Johnson in his Stanford Encyclopedia of Philosophy article on Kant's moral philosophy, of the procedure to be used to evaluate whether a maxim can be consistently universalized:

First, formulate a maxim that enshrines your reason for acting as you propose. Second, recast that maxim as a universal law of nature governing all rational agents, and so as holding that all must, by natural law, act as you yourself propose to act in these circumstances. Third, consider whether your maxim is even conceivable in a world governed by this law of nature. If it is, then, fourth, ask yourself whether you would, or could, rationally will to act on your maxim in such a world. If you could, then your action is morally permissible.

Back to your specific questions:
First, in the famous essay in which Kant discusses the murderer at the door example, he actually makes very little use of the Formula of Universal Law and its universalization test. But if we use that Formula to ascertain whether lying to the murderer is morally permissible, we must consider precisely what maxim is being tested. As you put it, there's a question of how to "categorize" an action, or in more Kantian terms, a question of what maxim a person considering lying to the murderer would be acting upon. Notice that "it's OK to lie" or "one should not assist murderers" do not even have the proper form of maxims. Neither are statements of reasons for action but instead moral statements about what a person may or ought to do. Again, we need a principle of the form "I will do act A in circumstances C to achieve end E". Notice that maxims often will be complex. And it is this complexity that has led many readers of Kant's essay to conclude that, Kant's remarks notwithstanding, his Formula of Universal Law does not necessarily disallow lying to the murderer. A maxim of the form "I will deceive a would-be murderer in order to prevent the death of his intended victim" seems like it could pass Kant's universalization test. (I'll leave that as an exercise to you.)

Long story short: Maxims are more complicated than is often recognized, and once this complexity is recognized, it is far from obvious that the conclusions Kant seems to draw in his famous essay are correct even by his own lights.

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