The question of the meaning (Sinn) of being (Sein) is the question which guides everything that is said in Being and Time and which determines the choice of topics to be examined. As we have already indicated, and as the very title of the work suggests, Heidegger thinks that the key to understanding being is time. Properly understood, time is the sense or meaning (Sinn) of being. But the book does not seem to be about being as such but about the being of particular beings or entities. So we have the detailed analyses of the being of those entities Heidegger calls Zeug (equipment): readiness-to-hand (Zuhandenheit); and the somewhat less detailed analyses of the being of things or substances: presence-at-hand (Vorhandenheit). But such analyses are all in the context of the analysis of the being of that entity which I myself am: Dasein. Properly understood, the analyses of readiness-to-hand and presence-at-hand are part of the analysis of the being of Dasein. This is because comportment to entities other than itself and understanding of the being of entities other than itself are integral to the being of Dasein: existence.

Being and Time not only provides an account of the basic structures of the being of Dasein (the existentials); it also claims to provide an answer to the question regarding the sense or meaning of the being of this entity. The being of Dasein is care (Sorge); and the sense or meaning of care is time.