The sub-chapter I am currently reading is called the ontology of temporality. Now according to dictionary.com an ontology is “An explicit formal specification of how to represent the objects, concepts and other entities that are assumed to exist in some area of interest and the relationships that hold among them”. At least that is the definition that seem to fit most with the way Sartre is using the term at the moment. So it is looking at the objects, concepts and entities associated with temporality or time. I don’t know why he didn’t just call it the nature of time. Sometimes I think he is making it more complicated just because he can.