Within empirical dream research, definitions of dreaming have been highly variable and often motivated by underlying theoretical background assumptions held by the theorist. Thus, the pure description of the explanandum, which should come first in any scientific inquiry, has perhaps been biased by a pre-existing theory as to what might count as the explanans—the entities, processes, and concepts that are supposed to explain the phenomenon. We will only briefly mention three approaches to defining (and explaining) dreams in the recent history of dream research, where the definition and description of the data seem to have been theoretically motivated.
The field of dream research was, in the 1970–1990s, a theoretically disunified field. The deep disagreements over finding a definition of “dreaming” that would be acceptable across the field were noted by Nielsen (2000, p. 853)
[T]here is currently no widely accepted or standardized definition of dreaming.
as well as by Hobson et al. (2000, p. 1019):
[…T]here is no clearly agreed upon definition of what a dream is […] and we are not even close to agreement.
Hobson’s (1988, 1997, 2001) own definition of dreaming is (or at least was in his earlier writings) a list of some features of dream experience. According to him, a dream is mentation during sleep that has most of the following features: hallucination, delusion, narrative structure, hyperemotionality, and bizarreness. This definition may be (and was) criticized as including only paradigmatic late-night REM dreams that are spontaneously remembered and on which our everyday stereotype of what dreams are like is based. This bias in the definition towards REM dreams might be seen to reflect the underlying theoretical idea or commitment, obvious in Hobson’s earlier theories, that dream phenomenology should be (reductively) explained by referring to the features of REM neurophysiology.
The opposing, cognitive–psychological view of the 1980s and 1990s conceptualized dreaming as a cognitive process that should be explained at the cognitive–psychological level (Foulkes 1985). References to the neurophysiological level were unnecessary. In that time and in the spirit of functionalism and classical cognitive science, the cognitive levels of description and explanation were in general seen to be completely independent of implementation levels, such as neurophysiology. Furthermore, dreaming was thought to occur in every stage of sleep, not only REM sleep, and rather than being full of bizarreness was mostly a credible replica of the waking world. Thus, according to the cognitive approach, an explanation of dreaming cannot be based on neurophysiological mechanisms in general, or for REM sleep on neurophysiology in particular. The explanation should be given at cognitive levels rather than neurobiological ones. Interestingly, it was probably Foulkes (1985) who first characterized dreams in terms of the idea and the concept of simulation. In 1985 he described dreams as credible world analogs, an organized form of consciousness that simulates what life is like in a nearly perfect manner.
A third theoretical definition of dreaming came from clinical dream research, and reflected the long and widespread idea in clinical psychology that dreams restore our emotional balance and have a psychotherapeutic function. Hartmann formulated this definition of dreaming most clearly, when he said that “Dreaming, like therapy, is the making of connections in a safe place” (1996, p. 13).
During recent years in dream research, the concept of simulation has become a widely accepted way of characterizing and defining dreaming, as well as a way of formulating theoretical ideas about the potential functions of dreaming. Thus, the idea that dreaming is a multimodal, complex, dynamic world-simulation in consciousness during sleep, may be a type of conception and definition of dreaming that many if not most dream researchers are ready to accept (Nielsen 2010). The various contents of dreams—their events and objects and characters—can be taken to be simulations of their real-world counterparts.
Taking Foulkes’s idea of dreams as credible world analogs and as the simulation of what life is like as a starting point for defining dreaming, Revonsuo (1995) formulated the Virtual Reality metaphor and later the TST (Threat Simulation Theory) of the evolutionary function of dreaming. This theory is built on two background assumptions, the first of which is precisely the definition of dreaming as “an organized simulation of the perceptual world” (Revonsuo 2000, p. 883). An additional, more specific assumption of this theory is that dream experience is specialized in particular in the simulation of threatening events: it tends to select and include various types of dangerous enemies and events and then simulates what it is like to perceive and recognize them (simulation of threat perception) as well as how to react and behaviourally respond to them (simulation of threat avoidance behaviours and strategies). Threat simulations appear in a paradigmatic and powerful form especially in nightmares, bad dreams, and post-traumatic dreams, but are also abundant in many other types of dreams such as everyday dreams, recurrent dreams, and in various parasomnias such as RBD (REM-Sleep Behaviour Disorder).
Domhoff (2007), who represents a similar psychological and content-analysis approach to dream research as Foulkes (1985), also characterizes dreams as mostly realistic and reasonable simulations of waking life. By emphasizing that, according to convincing empirical data from content-analysis studies of dreams, dream simulations are mostly realistic rather than overly bizarre and hyperemotional, Domhoff argues against the Hobsonian definition of dreaming as being full of bizarre contents.
Still, despite their disagreements, both camps now seem to accept the notion of simulation as a valid description of the core nature of dreaming. Hobson, in his new protoconsciousness theory of dreaming and REM sleep (2009), uses the concept of simulation to characterize the root phenomenon, protoconsciousness, from which both our waking and dreaming consciousness arise. According to Hobson, protoconsciousness is the simulated experiential reality or a virtual reality model of the world that the developing brain turns on during REM sleep even before birth, to prepare the conscious brain to simulate the external reality that it will encounter through the senses after birth. This model of the world is genetic, innate, and a human universal. Protoconsciousness acts as the template on which both waking and dreaming consciousness are built after birth. Thus, according to this theory, protoconscious dream consciousness—a very basic form of an internally simulated world—comes into being prior to waking consciousness, and is causally necessary for waking consciousness. As Hobson (2011, p. 30) puts it: “I REM, therefore I will be”. According to Hobson & Friston (2012), predictive coding is an underlying mechanism in the brain that produces predictive simulations of the world. Therefore, dreaming may also function as a preparatory simulation of the waking world; thus their idea is closely related to the other simulation-theories of dreaming (Hobson & Friston 2012).
In conclusion, while there still are disagreements about many details of dream content and function, there seems to be relatively widespread agreement that the definition of dreaming includes the idea of “simulation” of the waking world. The use of the concept of “simulation” to characterize dreaming has recently gained wide acceptance in the field. The simulation is variously characterized as the simulation of waking life, of waking reality, or of waking consciousness, and variously called by different authors a realistic world-simulation, a virtual reality, an immersive spatiotemporal model of the world, and so on—but despite the somewhat varying terminology, the different terms seem to describe the same basic idea. This conceptual unification is a significant step forward in the theoretical description and explanation of dreams. It paves the way for a more unified theory of dreaming.