Immanuel Kant 
The Metaphysical Elements of Ethics [EPUB ebook] 

If there exists on any subject a philosophy (that is, a system of rational knowledge based on concepts), then there must also be for this philosophy a system of pure rational concepts, independent of any condition of intuition, in other words, a metaphysic. It may be asked whether metaphysical elements are required also for every practical philosophy, which is the doctrine of duties, and therefore also for Ethics.