Although Ejzenštejn never writes about the ontology of cinema as such, the fundamental traits of such an ontology may be reconstructed by analysing some of the recurrent ideas that reappear throughout his large body of writings, from the essays on montage written during the 1920s, to the unfinished book projects of Metod (1932-48), Montage (1937), Nonindifferent Nature (1945-47), all the way up to the still unpublished notes for a General History of Cinema which he wrote in 1947-48. Throughout these writings cinema emerges as a medium based on montage whose goal is that of activating, emotionally and intellectually, the spectator. In order to pursue this goal, all the elements of the cinematic dispositif may be modified, and the most effective forms of montage disseminated throughout the history of the arts have to be located, analyzed, and taken as examples for the cinema of the future. Cinema thus emerges as a medium which is at the same time unstable (since its features may be modified at will) and diffused (since the principle of montage does not constitute its medium specificity but is rather a principle which manifests itself throughout the entire spectrum of media).