An Enquiry Concerning the Principles of Morals 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.