This lecture series from 1956, but recorded in astonishing detail by one of Deleuze's students, exhibits a surprising but otherwise untold-of moment of Deleuze's thought. In relation to its title, the text shifts subtly from the question 'What is Grounding?' which is given a Platonic, then Hegelian answer, to the question 'What is grounding for?' in response to which Deleuze considers deeply both Kierkegaard and Heidegger. For a philosopher so frequently accused of ahistoricality, this text ehibits the centrality of history to the origin of concepts, as he recognises the specific cultural provenance of his own concept of repetition in 19th century Denmark. Otherwise, how closely he follows Hegel in these lectures overturns readings of Deleuze, such as that of the Open Marxists, on which his ultimate rejection of Hegelianism is a product of his failure to ever read, let alone consider, the great German philosopher. These lectures show that Deleuze had an intimate understanding of the Phenomenology of Spirit as well as the Science of Logic, and in this context he even endorses the definition of true positivity as the negation of the negation.