The study of behavioral genetics seeks to answer two fundamental questions: To what extent are traits innate? And how can gene sequencing be utilized to predict behavior? This chapter begins with a critical evaluation of twin and adoption studies, describing the challenges of separating genetic influences from gene–environment interactions. The latter part explores modern genetic technologies, such as gene sequencing, and their application in identifying individuals who are at risk for developing neurological conditions, in predicting responses to treatment, and in employing polygenic screening for embryo selection.

This chapter provides a comprehensive overview of the neurotechnologies used to record and stimulate brain activity, from invasive techniques like optogenetics and intracranial electrodes to noninvasive methods such as electroencephalography and functional magnetic resonance imaging. It explains how these technologies are evaluated based on criteria like spatial resolution, temporal resolution, safety, and portability. With this framework, each technology is evaluated in terms of its power and constraints. This chapter highlights the trade-off between technological power and practical constraints, emphasizing the need for safer, more adaptable devices for both clinical and research purposes.

In the popular imagination, presences often mean spirits or ghosts. This chapter focuses on the notion of the spirit and how it connects to presence. During his research, the author met spiritual practitioners, diviners, psychics and mediums – many, but not all, considered themselves spiritualists. For them, there was no great mystery when it came to presence. It was spirit, pure and simple. The author suggests that felt presence could be shaped by our very own models of what is possible in the mind. We draw the boundaries; we decide who gets in and gets out. We might be the architects of where we stop and the other begins.

This chapter recounts the author’s interviews with people who hear voices that others cannot. This hearing of voices is connected to the central notion of ‘the presence feeling’. The author describes how in feeling that something is there, but not via our normal senses, we are sensing something impossible, ‘going beyond’ the ordinary sensory field. At the same time, these experiences don’t quite seem to fit the sensation of presence we have encountered already. They are beyond what someone could conceivably experience, but they do not pick out the social bit – the sense of someone being there. Presences, then, are not new. They have been there all along, with people trying to describe them but unable to pin them down or grasp why they come, why they are there.

Visual hallucinations are known to occur in both Parkinson’s and dementia with Lewy bodies (DLB). This chapter studies both conditions. The author posits that we have at least two different theories to consider. The body theory of felt presence reflects how we think the motor system works and provides a plausible explanation of how presences might be possible in particular situations and conditions. An alternative theory of presence is based on the broader idea of ‘predictive processing’. When we see, hear or feel things around us that aren’t there, our brain attempts to fill in the gaps based on some kind of ingrained expectation. That might not be something we are consciously expecting – it is more like a learned response. We can call this the expectation theory of presence. These models might not be mutually exclusive. Both of these theories could be playing a role in the presences of psychosis or Parkinson’s. They could potentially explain different examples of presence, or they could work in concert in some way, one laying the groundwork, the other offering a finishing touch.

This chapter begins with the story of Leven Brown, who rowed from Cadiz to Tobago. The author talks to Leven to understand more about the link between endurance and presence. There is a dearth of accounts of experiences that sound like presence for extended pursuits done solo: ultrarunners, free divers, long distance swimmers and sailors, for example. But what drives that connection? Is it just isolation, leading us to conjure companions? Is it about people being pushed to the extremes of their limits, mentally and physically? Or is it something more individual than that, something unique to the people who have these experiences? Some of the encounters reported are similar to the classic ‘Third Man’ experiences discussed in chapter 2. They come about in adverse situations, but they occur more in continuity with everyday life. The author is left with some questions: is it stress or adversity that prompts these experiences, or do they tend to occur for a certain kind of person? Do you have to be someone extraordinary already to enter this realm? And if you do need to be a certain kind of person, how can the experience ever be separated from the individual?

This chapter discusses the feeling of another person’s presence, which is often experienced in extreme weather conditions. The author discusses how, in such spaces, presences may not exactly be commonplace, but they are not unexpected. Mountaineers and climbers make up one community in which stories of presence are well-known and often shared. These presences are usually known under a different name, though: ‘The Third Man’ – the archetypal presence, the providential companion, the silent traveller, aiding those adrift in times of need. It is likely that such experiences have been happening for centuries, materialising out of blizzards, hillsides and glaciers, only to dissolve once more. In cases of climbing, the occurrence of hallucinations is often attributed to the effects of altitude.

The author begins this chapter by describing a study to test an unusual observation – that reading, in some senses, could be considered almost like a hallucinatory experience, and thus that there are parallels between imagination and hallucination. The author proposes a second kind of presence – those that are not out there in the world but in here with us and feel more real for it. If we think of the self as being defined in relation to others, it should not be a surprise that in some cases we can almost activate that other in our lives. It may even be a direct parallel to the ways in which disrupting the bodily self can bring forth bodily presences. The author suggests that it is a mistake to think that presences come from nowhere. They are other, and yet they are us – echoed, reflected and transformed. They are what can come forth when times become strange or pressured, when we lose track of where we begin and where the world ends. Their origin gives presences that feeling of significance and familiarity, for they are that scaffold when all is otherwise lost. They have been beside us all this time.

This chapter describes how digital interaction differs from physical – the feeling of being there with someone, sharing a common space, is lost. To comprehend how and why digital interactions feel so different, the author aims to understand how presence works in the virtual world. There is a specific definition that is applied to presence when it comes to virtual reality (VR): the sense of ‘being there’ in a computer-based environment. The author concludes by describing how ideas, methods and findings are changing fast in this field, as is the level of conversation about this experience. To understand felt presence in psychosis, we are ultimately going to need to explore both of these paths: of the body and the mind. There is not just one presence – there are others.