Immanuel Kant 
The Critique of Practical Reason 

'The Critique of Practical Reason’ is considered by many to be one of the most important works on the subject of moral philosophy ever written. In this work Immanuel Kant puts forth his philosophy of the categorical imperative, a rule for behavior that applies universally. Kant argues that a categorical imperative must guide whatever maxim one establishes for dealing with all decisions with moral implications. 'The Critique of Practical Reason’ is an excellent work on the philosophy of morality and a seminal work by one of the greatest philosophers of all time, Immanuel Kant.