Being and Time raises the problem of "authenticity," that is, the potentiality or otherwise for mortal "Dasein" to exist "fully" enough that it might actually understand being. Heidegger is clear throughout the book that nothing makes certain that "Dasein" is capable of this understanding. Finally, this question of the authenticity of individual "Dasein" cannot be separated from the "historicality" of "Dasein". On the one hand, "Dasein", as mortal, is "stretched along" between birth and death, and thrown into its world, that is, thrown into its "possibilities", possibilities which "Dasein" is charged with the task of assuming. On the other hand, "Dasein's" access to
Being and Time of what a hammer "is" is not determined by a theoretical understanding of its presence, but by the fact that it is something we need at the moment we wish to do hammering. Only a "later" understanding might come to contemplate a hammer "as" an object. The total understanding of being results from an explication of the implicit knowledge of being that inheres in "Dasein". Philosophy thus becomes a form of interpretation, but since there is no external reference point outside being from which to begin this interpretation, the question becomes to know in which way to proceed with this