This thesis pursues the question of how the authentic possibility of being constant can arise for Da-sein out of its lost inauthenticity by investigating what will provisionally be termed the environmentality of Da-sein. In creating a space for interpretive leeway, the term environmentality will remain distinct from the existential structure of worldliness, and from any particular factor of being-in-the-world specified in Division One of Being and Time. In assuming this direction, environmentality has its sight set on, or is awaiting, the way in which its proper potentiality for being-its-self returns to Da-sein after its interminable period of exile. Da-sein's authentic possibility will inevitably be presented to it in terms of its world, hence the preeminent fore-having guiding the term environmentality is how one world gets traded for another. For, the difference between constancy and non-self-constancy is categorical, and the modification of world entailed by individuation must itself be categorical: the world will change as a whole. The circumspection of attunement will be shown to grant environmentality its peculiar responsivity to possibility. The concept of ēthos, afforded in a reading of a passage in the Iliad, is specified as the character of authentic implacement from which the environmentality of Da-sein derives. The problem of individuation finds resolution in how ēthos is the authentic existentiell inflection of the structure of environmentality; by way of this specific affectivity (Befindlichkeit) Da-sein's ownmost potentiality-for-being-as-a-whole can be disclosed. The advent of the phenomenon of ēthos is disclosed in a reading of Beethoven's Pastoral Symphony, which lends the advantage of musical bracketing to draw out the heuristic indirection of Da-sein's shift in affectivity. The distinct possibility of affectivity named by ēthos has a tropological character which opens a space (aporregnumi) in which one is attracted in such a way that being attracted is accompanied by self-trust (resolve).