  Before a second key sense of the Heideggerian notion of world is revealed, some important detail can be added to the emerging picture. Heidegger points out that involvements are not uniform structures. Thus I am currently working with a computer (a with-which), in the practical context of my office (an in-which), in order to write this encyclopedia entry (an in-order-to), which is aimed towards presenting an introduction to Heidegger's philosophy (a towards-this), for the sake of my academic work, that is, for the sake of my being an academic (a for-the-sake-of-which). The final involvement here, the for-the-sake-of-which, is crucial, because according to Heidegger all totalities of involvements have a link of this type at their base. This forges a connection between (i) the idea that each moment in Dasein's existence constitutes a branch-point at which it chooses a way to be, and (ii) the claim that Dasein's projects and possibilities are essentially bound up with the ways in which other entities may become intelligible. This is because every for-the-sake-of-which is the base structure of an equipment-defining totality of involvements and reflects a possible way for Dasein to be (an academic, a carpenter, a parent, or whatever). Moreover, given that entities are intelligible only within contexts of activity that, so to speak, arrive with Dasein, this helps to explain Heidegger's claim (Being and Time 16: 107) that, in encounters with entities, the world is something with which Dasein is always already familiar. Finally, it puts further flesh on the phenomenological category of the un-ready-to-hand. Thus when I am absorbed in trouble-free typing, the computer and the role that it plays in my academic activity are transparent aspects of my experience. But if the computer crashes, I become aware of it as an entity with which I was working in the practical context of my office, in order to write an encyclopedia entry aimed towards presenting an introduction to Heidegger's philosophy. And I become aware of the fact that my behaviour is being organized for the sake of my being an academic. So disturbances have the effect of exposing totalities of involvements and, therefore, worlds. (For a second way in which worlds are phenomenologically ‘lit up’, see Heidegger's analysis of signs (Being and Time 17:107–114); for discussion, see Dreyfus 1990, 100–2, Cappuccio and Wheeler 2010.) 