An essay on pure ethics, with a theory of the motive Book

Hume is the moral philosopher who is most recognizable as a fellow modern human being. In his short autobiographical "My Own Life", he says that the Enquiry Concerning the Principles of Morals is "incomparably the best" of all his writings (though he admits that he isn't the one to judge that).

Reading this for the first time, I was surprised at how little "philosophy" is in it--by that, I mean how little complicated argumentation there is. Most of the arguments are short and direct: against someone who thinks that all of morality can be constructed out of self interest, Hume says that such a view is intuitively implausible, and gives some examples of difficult cases for such a theory to handle: admiring the moral qualities of an adversary, assigning moral praise or blame to historically different acts whose relation to present self interest is not at all clear, and so on. Of course there are responses that can be made on behalf of the views that Hume criticizes, but they look needlessly complex, sophistical and increasingly implausible when compared with his straightforward approach, which is to find the traits that we find agreeable and the traits we find disagreeable and see if they have anything in common. If they do, then that's likely to be the foundation of morals. There is no search for absolute certainty.

Not to give away the ending, but the foundation of morals turns out to be shared sentiment. One worry is that it's not as universal as he makes it out to be, but overall, this is about as enjoyable a work of moral philosophy as I've ever read (it's not as much fun as the Republic, but the view is saner).