Die deutsche Ideologie
In the "Economic and Philosophic Manuscripts of 1844" (1932), Karl Marx expressed the "Entfremdung" theory—of estrangement from the self. Philosophically, the theory of "Entfremdung" relies upon "The Essence of Christianity" (1841) by Ludwig Feuerbach, which states that the idea of a supernatural god has alienated the natural characteristics of the human being. Moreover, Max Stirner extended Feuerbach's analysis in "The Ego and its Own" (1845) that even the idea of 'humanity' is an alienating concept for individuals to intellectually consider in its full philosophic implication. Marx and Friedrich Engels responded to these philosophical propositions in "The German Ideology" (1845).