Failing to use it drives us into a state of heteronomy rather than autonomy and it is often defined in terms of treating others as ends in themselves.  
In Chapter 67 of Of Human Bondage, Philip Carey dismisses this concept as "perfect nonsense."  One of the major criticisms of it was Benjamin Constant's example of the "Enquiring Murderer."  Failing to use it drives us into a state of heteronomy rather than autonomy and it is often defined in terms of treating others as ends in themselves.  Originally expressed in Groundwork of the Metaphysics of Morals, it is usually stated as a dictum to act only according to those maxims which you can simultaneously will to become universal laws.  FTP, name this concept central to the moral philosophy of Kant.
In Chapter 67 of Of Human Bondage, Philip Carey dismisses this concept as "perfect nonsense."  
categorical imperative