6 1 Introduction: Divided Presence in Mixed Reality Presence is experienced strongly when we mostly attend to the currently present environment within and around the body (Waterworth and Waterworth 2001). The capacity we have for such attention depends on the amount of abstract conceptual processing a situation demands, as well as out own state of mental activation, because the capacity of our attention is strictly limited (see, e.g., Lavie et al. 2004;Lavie2005). As we process more abstract, conceptual information we can consciously sample fewer concrete aspects of the present situation, and so our sense of presence diminishes and we become relatively absent (Waterworth and Waterworth 2001; Waterworth et al. 2010), in a state of being mentally removed fromtheworldaroundus–whetherthisisphysicalorvirtual. Weneedtounderstandthis presence-absencedistinction if we are to understand howadigital virtual reality can evoke a coherent sense of presence, and why many mixed realities do not. Velmans (2000) describes how “What we normally call the ‘physical world’ just is what we experience. There is no additional experience of the world ‘in the mind or brain’”. Physical things are experienced as outside the body, in the external world, a process that Velmans calls “perceptual projection”. But, as he further points out “We also have ‘inner’ experiences such as verbal thoughts, images, feelings of knowing, experienced desires, and so on” and “they are reflexively experienced to be roughly where they are (in the head or brain)” (Velmans 2000, p. 110). Perceptual projection can occurs in response to both physical reality and virtual reality. As Velmans states: “Virtual reality systems in which one appears to interact with a (virtual) three-dimensional world in the absence of an actual (corresponding)world provide one of the best demonstrations of perceptual projection in action” (Velmans 2000, p. 231). Perceptual projection underlies our definition of presence as the feeling of being in an external (to the body) world, whereas absence is the feeling of being in an internal world, a world ‘in the head’. The distinction between internally- and externally-generated worlds becomes clear if we consider the difference between reading a gripping novel and acting in a convincingVR.Theworldofthenovelisdepictedinabstractform–thesymbolsof textual language printed on a page or presented on a screen. We must do conceptual worktorealize this world in our minds. A VR, in contrast, is depicted in a concrete form, and can be experienced in the ideal case without extra conceptual work and by exactly the same perceptual processes we draw on to interact with the physical world. Most importantly,virtual realities and physical realities can be shared.Other things being equal, a specific physical place that I visit is the same place when you visit it (event though your experience there will be somewhat different) and, in the same way, a specific virtual reality that I enter is the same one that you enter. We maymeeteachotherinavirtualreality or in a physical place. In contrast, the world I realize in my head when I read a novel is not the same as the one you realize, though it will have some similarities. It makes no sense to say that I will meet you in a place depicted in a novel. We can share external worlds in whichwefeelpresent, but we cannotshare imagined worlds in the same way. Thus media form determines the extent to which information is realized externally, and so is sharable, or internally (Waterworth et al. 2015). The key formal requirement