  According to Ingarden, the realism/idealism problem is fundamentally a metaphysical problem (about the actual existence of the so-called ‘real’ world and its relation to consciousness), but may be non-circularly approached via ontology by examining what the possible sorts of relation between consciousness and the world could be. In particular, Ingarden hoped that an ontological approach to the realism/idealism problem could lead to a solution by attempting to identify what the possible modes of being would be of the ‘real’ world and of consciousness, and how the two could possibly be related. This was the motive for his monumental work in ontology, The Controversy over the Existence of the World, designed to describe the different possible modes of being and their possible interrelations, with a view to narrowing down possible solutions to the realism/idealism problem. Unfortunately, the work was never fully completed (although the first two volumes were published and the third in progress at the time of Ingarden’s death), but the portions that exist nonetheless contain many important and detailed ontological analyses valuable in their own right as well as having the potential to contribute to the discussion of the realism/idealism controversy. Prominent among these is his distinction between formal, material, and existential ontologies, and distinguishing ‘modes of being’ as highest existential categories.