I heard about Kant's reasoning that lying that you return money or about the leads to contradiction in conception.
But how could he even prove that lying under any circumstances leads to contradiction in conception?
Let us assume some kind of soft extortionists. They threaten people to give them money but only if people have them. But they do not check if it is true since they are afraid of consequences (it can even result in murder and/or imprisonment).
Now let's assume that all people lie to them that they do not have money. Since they all lie, extortionists do no get money. Even if they are in doubt of people's words. After many unfortunate attempts they give up.
Isn't this a counterexample for "do not lie" duty? If so, how can Kantianism even prove that perfect duties even exist? And moreover the perfection of any particular duty.