  But why is the categorical imperative such a formal principle?  Why cannot it derive any of our more specific duties?  Rehberg's argument here becomes somewhat blurred and confused.  While his intention is to show that the categorical imperative is a purely formal principle, some of his arguments tend to demonstrate not its formality but the impossibility of acting according to it in the real empirical world. It is one thing to show that the categorical imperative is not practical because it cannot determine specific duties; it is another thing to show that it is not practical because no one in the real world could act according to it, whether it derives specific duties or not.  All told, Rehberg's polemic against Kant is an extraordinary mixture of insight and confusion.  We can best summarize it in the following points: