In The Art of Gratitude, Jeremy David Engels sketches a genealogy of gratitude from the ancient Greeks to the contemporary self-help movement. One of the most striking things about gratitude, Engels finds, is how consistently it is described using the language of indebtedness. A chief purpose of this, he contends, is to make us more comfortable living lives in debt, with the nefarious effect of pacifying the citizenry so we are less likely...