An Enquiry Concerning the Principles of Morals (EPM) is a book by Scottish enlightenment philosopher David Hume. In it, Hume argues (among other things) that the foundations of morals lie with sentiment, not reason.
An Enquiry Concerning the Principles of Morals is the enquiry subsequent to the Enquiry Concerning Human Understanding (EHU). Thus, it is often referred to as "the second Enquiry". It was originally published in 1751, three years after the first Enquiry. Hume first discusses ethics in A Treatise of Human Nature (in Book 3 - "Of Morals"). He later extracted and expounded upon the ideas he proposed there in his second Enquiry. In his short autobiographical work, My Own Life (1776), Hume states that his second Enquiry is "of all my writings, historical, philosophical, or literary, incomparably the best."