"Ressentiment" as a concept gained popularity with Friedrich Nietzsche's writings. Walter Kaufmann ascribes his use of the term in part to the absence of a proper equivalent term in the German language, contending that this absence alone "would be sufficient excuse for Nietzsche", if not for a translator. The term came to form a key part of his ideas concerning the psychology of the 'master–slave' question (articulated in "Beyond Good and Evil"), and the resultant birth of morality. Nietzsche's chief development of ressentiment came in his book "On The Genealogy of Morals"; see esp §§ 10–11). Earlier it had been used by Søren Kierkegaard. notably in his "".