Adorno sharply questions the ontotheological foundationalism in the thought of Heidegger and contends that this motif is present in Jaspers’ work on Nietzsche. Adorno’s ‘Jargon of Authenticity’ contains his most pointed critique of Heidegger and existentialist ontology. Nietzsche, Adorno contends, did not live long enough to experience the disgust with this jargon, which seeks to reconstruct an original meaning. As a ressentiment phenomenon par excellence, the jargon of authenticity gives, by Adorno’s account, a new meaning to Nietzsche’s “it does not smell good.” Like Adorno, Jaspers finds the work of Kierkegaard pivotal for the unfolding of modernity. For Jaspers, Nietzsche represented modernity in a somersaulting fashion. Nietzsche and Kierkegaard ran modernity into the ground, and “overcame it by living it through to the end.” Jaspers identified Nietzsche’s break with metaphysics as the end of modernity. The latter’s antisystematic questioning of reason recognizes Being only as infinite interpretation and engages, according to Jasper’s understanding, in unlimited and infinite reflection, which can neither exhaust nor stop itself. Kierkegaard and Nietzsche did not oppose reflection in order to annihilate it, but rather in order to overcome it by limitlessly engaging in it and mastering it. This mode of thought is an “endlessly active dialectic, the condition of freedom. It breaks out of every prison of the finite.”
Although Adorno is also thoroughly impressed by Kierkegaard and Nietzsche’s antisystematic and antimetaphysical efforts, he does not share Jaspers; view that they mark a definitive end to metaphysics or modernity. While he sees Nietzsche’s thought as completing enlightenment and in some respects as a turning point in the Western philosophical tradition, Adorno also criticizes Nietzsche for remaining caught within that same tradition. On the one hand, Adorno maintains that Nietzsche expresses the strongest argument against theology and metaphysics, while he claims on the other that Nietzsche’s critique never reaches the last stage of appeal. Nietzsche’s concepts of 'amor fait’, the 'Ubermensch’, and his view of women are but some examples of his unsuccessful escape from conventionality and tradition. Adorno makes the further claim that Nietzsche reverts to myth when he advocates, for instance, the concept of master-slave morality.
For Adorno, Jasper’s belief that Kierkegaard marks the overcoming of modernity overestimates the latter’s impact. Whereas Nietzsche, despite his setbacks, is a turning point in the history of ideas, Kierkegaard remains trapped in idealism. Although Kierkegaard’s works make the first step out of metaphysics, he does not escape them. In his habilitation thesis, which was published under the title 'Kierkegaard: Construction of the Aesthetic’, Adorno approaches Kierkegaard though a mode of criticism that simultaneously attempts to de(con)struct Kierkegaard’s work and rescue it. Adorno characterizes Kierkegaard’s philosophy as a form of late idealism and contends that Kierkegaard’s concept of objectless interiority or inwardness expresses yet another yearning for an absolute an transcendent truth. […] For Adorno, this retreat into interiority is unacceptable, because “the whole philosophy of inwardness, with its professed contempt for the world, is the last sublimation of the brutal, barbaric lore whereby he who was there first has the greatest rights; the priority of the self is as untrue as that of all who feel at home where they live.”
— Karin Bauer, ‘Adorno’s Nietzschean Narratives: Critiques of Ideology, Readings of Wagner'