In the article the author analyses Jean-Luc Nancy's attempt to transform the traditional aim and structure of fundamental ontology or 'first philosophy'. By radicalising the Heideggerian motif of 'Mitsein', being-with, Nancy introduces irreducible plurality and diversity into the structure of ontological fundament as well as into the very expositional structure of ontology, which becomes a 'co-ontology' or a 'coexistential analytic of being-with'. He focuses on the issues of 'the singular

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... ' and 'origins' to reconstruct the process of this transformation of ontology and indicate its various consequences. One of them is that ontology of being-with can only exist in a transitional, ever-transforming way, in singularly plural forms of regional co-ontologies which are to be constantly re-shaped to take up the challenge eventfully posed by different spheres of coexistence.