In animals, the brain is the control center of the central nervous system, responsible for behavior. In mammals, the brain is located in the head, protected by the skull and close to the primary sensory apparatus of vision, hearing, equilibrioception (balance), sense of taste, and olfaction (smell).

While all vertebrates have a brain, most invertebrates have either a centralized brain or collections of individual ganglia. Some animals such as cnidarians and echinoderms do not have a centralized brain, and instead have a decentralized nervous system, while animals such as sponges lack both a brain and nervous system entirely.

Brains can be extremely complex. For example, the human brain contains roughly 100 billion neurons, each linked to as many as 10,000 other neurons.

Human brain

History

Main article: History of the brain

Early views on the function of the brain regarded it as little more than cranial stuffing. In Ancient Egypt, from the late Middle Kingdom onwards, in preparation for mummification, the brain was regularly removed, for it was the heart that was assumed to be the seat of intelligence. According to Herodotus, during the first step of mummification, "The most perfect practice is to extract as much of the brain as possible with an iron hook, and what the hook cannot reach is mixed with drugs." Over the next five-thousand years, this view came to be reversed; the brain is now known to be seat of intelligence, although idiomatic variations of the former remain, as in memorizing something "by heart".

The first thoughts on the field of psychology came from ancient philosophers, such as Aristotle. As thinkers became more in tune with biomedical research over time, as was the case with medieval psychologists such as Alhazen and Avicenna for example, the concepts of experimental psychology and clinical psychology began emerging. From that point, different branches of psychology emerged with different individuals creating new ideas, with modern psychologists such as Freud and Jung contributing to the field.

Mind and brain

The distinction between the mind and the brain is fundamental in philosophy of mind. The mind-body problem is one of the central problems in the history of philosophy. The brain is the physical and biological matter contained within the skull, responsible for electrochemical neuronal processes. The mind, in contrast, consists in mental attributes, such as beliefs, desires, perceptions, and so on. There are scientifically demonstrable correlations between mental events and neuronal events; the philosophical question is whether these phenomena are identical, at least partially distinct, or related in some other way.

Philosophical positions on the mind-body problem fall into two main categories. The first category is dualism, according to which the mind exists independently of the brain. Dualist theories are further divided into substance dualism and property dualism. René Descartes is perhaps the most prominent substance dualist, while property dualism is more popular among contemporary dualists like David Chalmers. The second category is materialism, according to which mental phenomena are identical to neuronal phenomena. A third category of view, idealism, claims that only mental substances and phenomena exist. This view, most prominently held by 18th century Irish philosopher Bishop George Berkeley, has few contemporary adherents.

Both dualism and materialism face serious philosophical challenges. Dualism requires admitting non-physical substances or properties into ontology, which is in apparent conflict with the scientific world view. Materialism, on the other hand, must provide an explanation of how two seemingly different kinds of phenomena – the mental and the physical – could be identical. This is particularly challenging in that mental phenomena have certain characteristics – particularly intentionality and phenomenal character – that cannot (at least currently) be explained satisfactorily by a purely physical analysis of the brain.

Comparative anatomy

Three groups of animals have notably complex brains: the arthropods (insects, crustaceans, arachnids, and others), the cephalopods (octopuses, squids, and similar mollusks), and the craniates (vertebrates and hagfish). The brain of arthropods and cephalopods arises from twin parallel nerve cords that extend through the body of the animal. Arthropods have a central brain with three divisions and large optical lobes behind each eye for visual processing.

The brain of craniates develops from the anterior section of a single dorsal nerve cord, which later becomes the spinal cord. In craniates, the brain is protected by the bones of the skull.

Mammals have a six-layered neocortex (or homotypic cortex, neopallium), in addition to having some parts of the brain that are allocortex. In mammals, increasing convolutions of the brain are characteristic of animals with more advanced brains. These convolutions provide a larger surface area for a greater number of neurons while keeping the volume of the brain compact enough to fit inside the skull. The folding allows more grey matter to fit into a smaller volume. The folds are called sulci, while the spaces between the folds are called gyri.

In birds, the part of the brain that functionally corresponds to the neocortex is called nidopallium and derives from a different part of the brain. Some birds (like corvids and parrots), are thought by some to have high intelligence, but even in these, the brain region that forms the mammalian neocortex is in fact almost entirely absent.

Although the general histology of the brain is similar from person to person, the structural anatomy can differ. Apart from the gross embryological divisions of the brain, the location of specific gyri and sulci, primary sensory regions, and other structures differs between species.

Insects

In insects, the brain has four parts, the optical lobes, the protocerebrum, the deutocerebrum, and the tritocerebrum. The optical lobes are behind each eye and process visual stimuli. The protocerebrum contains the mushroom bodies, which respond to smell, and the central body complex. In some species such as bees, the mushroom body receives input from the visual pathway as well. The deutocerebrum includes the antennal lobes, which are similar to the mammalian olfactory bulb, and the mechanosensory neuropils which receive information from touch receptors on the head and antennae. The antennal lobes of flies and moths are quite complex.

Cephalopods

In cephalopods, the brain has two regions: the supraesophageal mass and the subesophageal mass, separated by the esophagus. The supra- and subesophageal masses are connected to each other on either side of the esophagus by the basal lobes and the dorsal magnocellular lobes. The large optic lobes are sometimes not considered to be part of the brain, as they are anatomically separate and are joined to the brain by the optic stalks. However, the optic lobes perform much visual processing, and so functionally are part of the brain.

Mammals and other vertebrates

The telencephalon (cerebrum) is the largest region of the mammalian brain. This is the structure that is most easily visible in brain specimens, and is what most people associate with the "brain". In humans and several other animals, the fissures (sulci) and convolutions (gyri) give the brain a wrinkled appearance. In non-mammalian vertebrates with no cerebrum, the metencephalon is the highest center in the brain. Because humans walk upright, there is a flexure, or bend, in the brain between the brain stem and the cerebrum. Other vertebrates do not have this flexure. Generally, comparing the locations of certain brain structures between humans and other vertebrates often reveals a number of differences.

Behind (or in humans, below) the cerebrum is the cerebellum. The cerebellum is known to be involved in the control of movement, and is connected by thick white matter fibers (cerebellar peduncles) to the pons. The cerebrum has two cerebral hemispheres. The cerebellum also has hemispheres. The telencephalic hemispheres are connected by the corpus callosum, another large white matter tract. An outgrowth of the telencephalon called the olfactory bulb is a major structure in many animals, but in humans and other primates it is relatively small.

Vertebrate nervous systems are distinguished by bilaterally symmetrical encephalization. Encephalization refers to the tendency for more complex organisms to gain larger brains through evolutionary time. Larger vertebrates develop a complex, layered and interconnected neuronal circuitry. In modern species most closely related to the first vertebrates, brains are covered with gray matter that has a three-layer structure (allocortex). Their brains also contain deep brain nuclei and fiber tracts forming the white matter. Most regions of the human cerebral cortex have six layers of neurons (neocortex).

Vertebrate brain regions

(See related article at List of regions in the human brain)

According to the hierarchy based on embryonic and evolutionary development, chordate brains are composed of the three regions that later develop into five total divisions:

• Rhombencephalon (hindbrain)
  • Myelencephalon
  • Metencephalon
• Mesencephalon (midbrain)
• Prosencephalon (forebrain)
  • Diencephalon
  • Telencephalon

The brain can also be classified according to function, including divisions such as:

• Limbic system
• Sensory systems
  • Visual system
  • Olfactory system
  • Gustatory system
  • Auditory system
  • Somatosensory system
• Motor system
• Associative areas

In recent years it was realized that certain birds have developed high intelligence entirely convergently from mammals such as humans. Hence, the functional areas of the avian brain have been redefined by the Avian Brain Nomenclature Consortium. See also Bird intelligence.

Humans

Main article: Human brain

The structure of the human brain differs from that of other animals in several important ways. These differences allow for many abilities over and above those of other animals, such as advanced cognitive skills. Human encephalization is especially pronounced in the neocortex, the most complex part of the cerebral cortex. The proportion of the human brain that is devoted to the neocortex—especially to the prefrontal cortex—is larger than in all other mammals (indeed larger than in all animals, although only in mammals has the neocortex evolved to fulfill this kind of function).

Humans have unique neural capacities, but much of their brain structure is similar to that of other mammals. Basic systems that alert the nervous system to stimulus, that sense events in the environment, and monitor the condition of the body are similar to those of even non-mammalian vertebrates. The neural circuitry underlying human consciousness includes both the advanced neocortex and prototypical structures of the brainstem. The human brain also has a massive number of synaptic connections allowing for a great deal of parallel processing.

The human brain is also the largest organ in the Nervous System.

Neurobiology

The brain is composed of two broad classes of cells, neurons and glia, both of which contain several different cell types which perform different functions. Interconnected neurons form neural networks (or neural ensembles). These networks are similar to man-made electrical circuits in that they contain circuit elements (neurons) connected by biological wires (nerve fibers). These do not form simple one-to-one electrical circuits like many man-made circuits, however. Typically neurons connect to at least a thousand other neurons. These highly specialized circuits make up systems which are the basis of perception, different types of action, and higher cognitive function.

Structure

Structure of a typical neuron

Neuron
Dendrite Soma Axon Nucleus Node of Ranvier Axon Terminal Schwann cell Myelin sheath

Neurons are the cells that convey information to other cells; these constitute the essential class of brain cells.

In addition to neurons, the brain contains glial cells in a roughly 10:1 proportion to neurons. Glial cells ("glia" is Greek for “glue”) form a support system for neurons. They create the insulating myelin, provide structure to the neuronal network, manage waste, and clean up neurotransmitters. Most types of glia in the brain are present in the entire nervous system. Exceptions include the oligodendrocytes which myelinate neural axons (a role performed by Schwann cells in the peripheral nervous system). The myelin in the oligodendrocytes insulates the axons of some neurons. White matter in the brain is myelinated neurons, while grey matter contains mostly cell soma, dendrites, and unmyelinated portions of axons and glia. The space between neurons is filled with dendrites as well as unmyelinated segments of axons; this area is referred to as the neuropil.

In mammals, the brain is surrounded by connective tissues called the meninges, a system of membranes that separate the skull from the brain. This three-layered covering is composed of (from the outside in) the dura mater, arachnoid mater, and pia mater. The arachnoid and pia are physically connected and thus often considered as a single layer, the pia-arachnoid. Below the arachnoid is the subarachnoid space which contains cerebrospinal fluid, a substance that protects the nervous system. Blood vessels enter the central nervous system through the perivascular space above the pia mater. The cells in the blood vessel walls are joined tightly, forming the blood-brain barrier which protects the brain from toxins that might enter through the blood.

The brain is bathed in cerebrospinal fluid (CSF), which circulates between layers of the meninges and through cavities in the brain called ventricles. It is important both chemically for metabolism and mechanically for shock-prevention. For example, the human brain weighs about 1-1.5 kg or about 2-3 lb. The mass and density of the brain are such that it will begin to collapse under its own weight if unsupported by the CSF. The CSF allows the brain to float, easing the physical stress caused by the brain’s mass.

Dreams are the images, thoughts and feelings experienced while asleep, particularly strongly associated with rapid eye movement sleep. The contents and biological purposes of dreams are not fully understood, though they have been a topic of speculation and interest throughout recorded history. The scientific study of dreams is known as oneirology.

Neurology of sleep and dreams

Main article: REM sleep

There is no universally agreed biological definition of dreaming. General observation shows that dreams are strongly associated with rapid eye movement (REM) sleep, during which an electroencephalogram shows brain activity to be most like wakefulness. Participant-nonremembered dreams during non-REM sleep are normally more mundane in comparison. During a typical lifespan, a human spends a total of about six years dreaming (which is about 2 hours each night). It is unknown where in the brain dreams originate, if there is a single origin for dreams or if multiple portions of the brain are involved, or what the purpose of dreaming is for the body or mind.

During REM sleep, the release of certain neurotransmitters is completely suppressed. As a result, motor neurons are not stimulated, a condition known as REM atonia. This prevents dreams from resulting in dangerous movements of the body.

Discovery of REM

In 1953 Eugene Aserinsky discovered REM sleep while working in the surgery of his PhD advisor. Aserinsky noticed that the sleepers' eyes fluttered beneath their closed eyelids, later using a polygraph machine to record their brain waves during these periods. In one session he awakened a subject who was crying and wailing out during REM and confirmed his suspicion that dreaming was occurring. In 1953 Aserinsky and his advisor published the ground-breaking study in Science.

Dream theories

Activation-synthesis

In 1976, J. Allan Hobson and Robert McCarley proposed a new theory that changed dream research, challenging the previously held Freudian view of dreams as unconscious wishes to be interpreted. The activation synthesis theory asserts that the sensory experiences are fabricated by the cortex as a means of interpreting chaotic signals from the pons. They propose that in REM sleep, the ascending cholinergic PGO (ponto-geniculo-occipital) waves stimulate higher midbrain and forebrain cortical structures, producing rapid eye movements. The activated forebrain then synthesizes the dream out of this internally generated information. They assume that the same structures that induce REM sleep also generate sensory information.

Hobson and McCarly's 1976 research suggested that the signals interpreted as dreams originated in the brain stem during REM sleep. However, research by Mark Solms suggests that dreams are generated in the forebrain, and that REM sleep and dreaming are not directly related. While working in the neurosurgery department at hospitals in Johannesburg and London, Solms had access to patients with various brain injuries. He began to question patients about their dreams and confirmed that patients with damage to the parietal lobe stopped dreaming; this finding was in line with Hobson's 1977 theory. However, Solms did not encounter cases of loss of dreaming with patients having brain stem damage. This observation forced him to question Hobson's prevailing theory which marked the brain stem as the source of the signals interpreted as dreams. Solms viewed the idea of dreaming as a function of many complex brain structures as validating Freudian dream theory, an idea that drew criticism from Hobson.

Continual-activation

Combining Hobson's activation synthesis hypothesis with Solms's findings, the continual-activation theory of dreaming presented by Jie Zhang proposes that dreaming is a result of brain activation and synthesis; at the same time, dreaming and REM sleep are controlled by different brain mechanisms. Zhang hypothesizes that the function of sleep is to process, encode, and transfer the data from the temporary memory to the long-term memory, though there is not much evidence backing up this so-called "consolidation." Non-REM sleep processes the conscious-related memory (declarative memory), and REM sleep processes the unconscious related memory (procedural memory).

Zhang assumes that during REM sleep, the unconscious part of a brain is busy processing the procedural memory; meanwhile, the level of activation in the conscious part of the brain will descend to a very low level as the inputs from the sensory are basically disconnected. This will trigger the "continual-activation" mechanism to generate a data stream from the memory stores to flow through the conscious part of the brain. Zhang suggests that this pulse-like brain activation is the inducer of each dream. He proposes that, with the involvement of the brain associative thinking system, dreaming is, thereafter, self-maintained with the dreamer's own thinking until the next pulse of memory insertion. This explains why dreams have both characteristics of continuity (within a dream) and sudden changes (between two or more dreams).

Dreams and memory

Eugen Tarnow suggests that dreams are ever-present excitations of long-term memory, even during waking life. The strangeness of dreams is due to the format of long-term memory, reminiscent of Penfield & Rasmussen’s findings that electrical excitations of the cortex give rise to experiences similar to dreams. During waking life an executive function interprets long term memory consistent with reality checking. Tarnow's theory is a reworking of Freud's theory of dreams in which Freud's unconscious is replaced with the long-term memory system and Freud's “Dream Work” describes the structure of long-term memory.

Hippocampus and memory

A 2001 study showed evidence that illogical locations, characters, and dream flow may help the brain strengthen the linking and consolidation of semantic memories. These conditions may occur because, during REM sleep, the flow of information between the hippocampus and neocortex is reduced. Increasing levels of the stress hormone cortisol late in sleep (often during REM sleep) cause this decreased communication. One stage of memory consolidation is the linking of distant but related memories. Payne and Nadel hypothesize that these memories are then consolidated into a smooth narrative, similar to a process that happens when memories are created under stress

Functional hypotheses

There are many hypotheses about the function of dreams, including:

• During the night there may be many external stimuli bombarding the senses, but the mind interprets the stimulus and makes it a part of a dream in order to ensure continued sleep. The mind will, however, awaken an individual if they are in danger or if trained to respond to certain sounds, such as a baby crying.
• Dreams allow the repressed parts of the mind to be satisfied through fantasy while keeping the conscious mind from thoughts that would suddenly cause one to awaken from shock.

• Freud suggested that bad dreams let the brain learn to gain control over emotions resulting from distressing experiences.
• Jung suggested that dreams may compensate for one-sided attitudes held in waking consciousness.
• Ferenczi proposed that the dream, when told, may communicate something that is not being said outright.
• Dreams are like the cleaning-up operations of computers when they are off-line, removing parasitic nodes and other "junk" from the mind during sleep.
• Dreams create new ideas through the generation of random thought mutations. Some of these may be rejected by the mind as useless, while others may be seen as valuable and retained. Blechner calls this the theory of "Oneiric Darwinism."
• Dreams regulate mood.
• Hartmann says dreams may function like psychotherapy, by "making connections in a safe place" and allowing the dreamer to integrate thoughts that may be dissociated during waking life.
• More recent research by Griffin has led to the formulation of the 'expectation fulfillment theory of dreaming', which suggests that dreaming metaphorically completes patterns of emotional expectation and lowers stress levels.
• Coutts hypothesizes that dreams modify and test mental schemas during sleep during a process he calls emotional selection, and that only schema modifications that appear emotionally adaptive during dream tests are selected for retention, while those that appear maladaptive are abandoned or further modified and tested.

Dreams and psychosis

A number of thinkers have commented on the similarities between the phenomenology of dreams and that of psychosis. These include thought disorder, flattened or inappropriate affect (emotion), and hallucination. Among philosophers, Kant, for example, wrote that ‘the lunatic is a wakeful dreamer’. Schopenhauer said: ‘A dream is a short-lasting psychosis, and a psychosis is a long-lasting dream.’In the field of psychoanalysis, Freud wrote: ‘A dream then, is a psychosis’,and Jung: ‘Let the dreamer walk about and act like one awakened and we have the clinical picture of dementia praecox.’

McCreery has sought to explain these similarities by reference to the fact, documented by Oswald, that sleep can supervene as a reaction to extreme stress and hyper-arousal. McCreery adduces evidence that psychotics are people with a tendency to hyper-arousal, and suggests that this renders them prone to what Oswald calls ‘micro-sleeps’ during waking life. He points in particular to the paradoxical finding of Stevens and Darbyshire that patients suffering from catatonia can be roused from their seeming stupor by the administration of sedatives rather than stimulants.

Cultural history

Dreams have a long history both as a subject of conjecture and as a source of inspiration. Throughout their history, people have sought meaning in dreams or divination through dreams. They have been described physiologically as a response to neural processes during sleep, psychologically as reflections of the subconscious, and spiritually as messages from God or predictions of the future. Many cultures practiced dream incubation, with the intention of cultivating dreams that were prophetic or contained messages from the divine.

Judaism has a traditional ceremony that is to follow particularly disturbing dreams.

Dream content

From the 1940s to 1985, Calvin S. Hall collected more than 50,000 dream reports at Western Reserve University. In 1966 Hall and Van De Castle published The Content Analysis of Dreams in which they outlined a coding system to study 1,000 dream reports from college students. It was found that people all over the world dream of mostly the same things. Hall's complete dream reports became publicly available in the mid-1990s by Hall's protégé William Domhoff, allowing further different analysis.

Personal experiences from the last day or week are frequently incorporated into dreams.

Emotions

The most common emotion experienced in dreams is anxiety. Negative emotions are more common than positive feelings. The U.S. ranks the highest amongst industrialized nations for aggression in dreams with 50 percent of U.S. males reporting aggression in dreams, compared to 32 percent for Dutch men.

Sexual content

The Hall data analysis shows that sexual dreams occur no more than 10 percent of the time and are more prevalent in young to mid teens. Another study showed that 8% of men's and women's dreams have sexual content. In some cases, sexual dreams may result in orgasm or nocturnal emission. These are commonly known as wet dreams.

Recurring dreams

While the content of most dreams is dreamt only once, many people experience recurring dreams—that is, the same dream narrative is experienced over different occasions of sleep. Up to 70% of females and 65% of males report recurrent dreams.

Common themes

Content-analysis studies have identified common reported themes in dreams. These include: situations relating to school, being chased, running slowly in place, sexual experiences, falling, arriving too late, a person now alive being dead, teeth falling out, flying, embarrassing moments, failing an examination, not being able to move, not being able to focus vision and car accidents. Twelve percent of people dream only in black and white.

Relationship with mental illness

There is evidence that certain medical conditions (normally only neurological conditions) can impact dreams. For instance, people with synesthesia have never reported black-and-white dreaming, and often have a difficult time imagining the idea of dreaming in only black and white.

Therapy for recurring nightmares (often associated with posttraumatic stress disorder) can include imagining alternative scenarios that could begin at each step of the dream.

Dream interpretation

Dreams were historically used for healing (as in the asclepieions found in the ancient Greek temples of Asclepius) as well as for guidance or divine inspiration. Some Native American tribes used vision quests as a rite of passage, fasting and praying until an anticipated guiding dream was received, to be shared with the rest of the tribe upon their return.

In modern times, both Sigmund Freud and Carl Jung identified dreams as an interaction between the unconscious and the conscious. They also assert together that the unconscious is the dominant force of the dream, and in dreams it conveys its own mental activity to the perceptive faculty. While Freud felt that there was an active censorship against the unconscious even during sleep, Jung argued that the dream's bizarre quality is an efficient language, comparable to poetry and uniquely capable of revealing the underlying meaning.

Fritz Perls presented his theory of dreams as part of the holistic nature of Gestalt therapy. Dreams are seen as projections of parts of the self that have been ignored, rejected, or suppressed. Jung argued that one could consider every person in the dream to represent an aspect of the dreamer, which he called the subjective approach to dreams. Perls expanded this point of view to say that even inanimate objects in the dream may represent aspects of the dreamer. The dreamer may therefore be asked to imagine being an object in the dream and to describe it, in order to bring into awareness the characteristics of the object that correspond with the dreamer's personality.

Other associated phenomena

[Lucid dreaming

Lucid dreaming is the conscious perception of one's state while dreaming. In this state a person usually has control over characters and the environment of the dream as well as the dreamer's own actions within the dream. The occurrence of lucid dreaming has been scientifically verified.

"Oneironaut" is a term sometimes used for those who explore the world of dreams. For example, dream researcher Stephen LaBerge uses the term. It is often associated with lucid dreaming in particular.

Dreams of absent-minded transgression

Dreams of absent-minded transgression (DAMT) are dreams wherein the dreamer absentmindedly performs an action that he or she has been trying to stop (one classic example is of a quitting smoker having dreams of lighting a cigarette). Subjects who have had DAMT have reported waking with intense feelings of guilt. One study found a positive association between having these dreams and successfully stopping the behavior.

Dreaming and the "real world"

Dreams can link to actual sensations, such as the incorporation of environmental sounds into dreams such as hearing a phone ringing in a dream while it is ringing in reality, or dreaming of urination while wetting the bed. Except in the case of lucid dreaming, people dream without being aware that they are doing so. Some philosophers have concluded that what we think as the "real world" could be or is an illusion (an idea known as the skeptical hypothesis about ontology). The first recorded mention of the idea was by Zhuangzi, and was also discussed in Hinduism; Buddhism makes extensive use of the argument in its writings. It was formally introduced to western philosophy by Descartes in the 17th century in his Meditations on First Philosophy.

Recalling dreams

The recall of dreams is extremely unreliable, though it is a skill that can be trained. Dreams can usually be recalled if a person is awakened while dreaming. Women tend to have more frequent dream recall than men. Dreams that are difficult to recall may be characterized by relatively little affect, and factors such as salience, arousal, and interference play a role in dream recall. A dream journal can be used to assist dream recall, for psychotherapy or entertainment purposes.

Déjà vu

The theory of déjà vu dealing with dreams indicates that the feeling of having previously seen or experienced something could be attributed to having dreamt about a similar situation or place, and forgetting about it until one seems to be mysteriously reminded of the situation or place while awake.

Dream pre-programming

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Dream pre-programming is a hypnotic practice used among some medical and stage hypnotists. It allows the hypnotist to control (or let the patient control) their own dreams. One way that a hypnotist will use this is by telling the person that when they fall asleep that they see a button. And that if they want to enter "DreamScape" that they should press that button. Then they will enter a world just like Earth, but they will have complete control. They will control things with their mind. Dream pre-programming can also help someone for a test or a big event in life. The hypnotist would make the subject dream that event as going perfect, so the subject will get a level of confidence.

Dream incorporation

In one use of the term, "dream incorporation" is a phenomenon whereby an external stimulus, usually an auditory one, becomes a part of a dream, eventually then awakening the dreamer. There is a famous painting by Salvador Dalí that depicts this concept, titled "Dream Caused by the Flight of a Bee around a Pomegranate a Second Before Awakening" (1944).

The term "dream incorporation" is also used in research examining the degree to which preceding daytime events become elements of dreams. Recent studies suggest that events in the day immediately preceding, and those about a week before, have the most influence .

Reality, in everyday usage, means "the state of things as they actually exist". The term reality, in its widest sense, includes everything that is, whether or not it is observable or comprehensible. Reality in this sense may include both being and nothingness, whereas existence is often restricted to being (compare with nature).
In other words, "reality", as a philosophical category includes the
formal concept of "nothingness" and articulations and combinations of
it with other concepts (those possessing extension in physical objects
or processes for example).

In the strict sense of western philosophy, there are levels or gradation to the nature and conception of reality. These levels include, from the most subjective to the most rigorous: phenomenological reality, truth, fact, and axiom.

Phenomenological reality

On a much broader and more subjective level, the private
experiences, curiosity, inquiry, and selectivity involved in the
personal interpretation of an event shapes reality as seen by one and
only one individual and hence is called phenomenological.
This form of reality might be common to others as well, but at times
could also be so unique to oneself as to be never experienced or agreed
upon by any one else. Much of the kind of experience deemed spiritual
occurs on this level of reality. From a phenomenological perspective,
reality is that which is phenomenally real and unreality is
nonexistent. Individual perception can be based upon an individual's
personality, focus and style of attribution, causing him or her to see
only what he or she wants to see or believes to be true.

A non-circular definition

Reality: A non-derivative experience

] Truth

Main article: Truth

According to the less realist trends in philosophy, such as postmodernism/post-structuralism,
truth is subjective. When two or more individuals agree upon the
interpretation and experience of a particular event, a consensus about
an event and its experience begins to be formed. This being common to a
few individuals or a larger group, then becomes the 'truth' as seen and
agreed upon by a certain set of people — the consensus reality. Thus one particular group
may have a certain set of agreed truths, while another group might have
a different set of consensual 'truths'. This lets different communities and societies have varied and extremely different notions of reality and truth of the external world. The religion and beliefs of people or communities are a fine example of this level of socially constructed
'reality'. Truth cannot simply be considered truth if one speaks and
another hears because individual bias and fallibility challenge the
idea that certainty or objectivity are easily grasped. For Anti-realists,
the inaccessibility of any final, objective truth means that there is
no truth beyond the socially-accepted consensus. (Although this means
there are truths, not truth).

For realists, the world is a set of definite facts, which obtain independently of humans ("The world is all that is the case" — Tractatus Logico-Philosophicus), and these facts are the final arbiter of truth. Michael Dummett expresses this in terms of the principle of bivalence^[2]: Lady Macbeth had three children or she did not; a tree falls or it does not. A statement will be true if it corresponds to these facts — even if the correspondence cannot be established. Thus the dispute between the realist and anti-realist conception of truth hinges on reactions to the epistemic accessibility (knowability, graspability) of facts.

Fact

Main article: Fact

A fact or factual entity is a phenomenon that is perceived as an
elemental principle. It is rarely one that could be subject to personal
interpretation. Instead, it is most often an observed phenomenon of the
natural world. The proposition 'viewed from most places on Earth, the
sun rises in the east', is a fact. It is a fact for people belonging to
any group or nationality, regardless of which language they speak or
which part of the hemisphere they come from. The Galilean proposition in support of the Copernican theory, that the sun is the center of the solar system is one that states the fact of the natural world.
However, during his lifetime Galileo was ridiculed for that factual
proposition, because far too few people had a consensus about it in
order to accept it as a truth. Fewer propositions are factual in
content in the world, as compared to the many truths shared by various
communities, which are also fewer than the innumerable individual worldviews. Much of scientific exploration, experimentation, interpretation and analysis is done on this level.

This view of reality is well expressed by Philip K. Dick's statement that "Reality is that which, when you stop believing in it, doesn't go away."

What reality might not be

"Reality," the concept, is contrasted with a wide variety of other
concepts, largely depending upon the intellectual discipline. It can
help us to understand what we mean by "reality" to note that what we
say is not real because we see it through different
perspectives, therefore there is no basis for reality. But usually if
there is no original and related proofs, it isn't reality.

In philosophy, reality is contrasted with nonexistence (penguins do exist; so they are real) and mere possibility
(a mountain made of gold is merely possible, but is not known to be
real—that is, actual rather than possible—unless one is discovered).
Sometimes philosophers speak as though reality is contrasted with existence
itself, though ordinary language and many other philosophers would
treat these as synonyms. They have in mind the notion that there is a kind of reality — a mental or intentional reality, perhaps — that imaginary objects, such as the aforementioned golden mountain, have. Alexius Meinong
is famous, or infamous, for holding that such things have so-called
subsistence, and thus a kind of reality, even while they do not
actually exist. Most philosophers find the very notion of "subsistence"
mysterious and unnecessary, and one of the shibboleths and starting points of 20th century analytic philosophy has been the forceful rejection of the notion of subsistence — of "real" but nonexistent objects.

Some schools of Buddhism hold that reality is something void of description, the formless which forms all illusions or maya.
Buddhists hold that we can only discuss objects which are not reality
itself and that nothing can be said of reality which is true in any
absolute sense. Discussions of a permanent self are necessarily about
the reality of self which cannot be pointed to nor described in any
way. Similar is the Taoist saying, that the Tao that can be named is not the true Tao, or way.

It is worth saying at this point that many philosophers are not content with saying merely what reality is not — some of them have positive theories of what broad categories of objects are real, in addition. See ontology as well as philosophical realism; these topics are also briefly treated below.

In ethics, political theory, and the arts, reality is often contrasted with what is "ideal."

One of the fundamental issues in ethics is called the is-ought problem,
and it can be formulated as follows: "Given our knowledge of the way
the world 'is,' how can we know the way the world 'ought to be'?" Most
ethical views hold that the world we live in (the real world) is not ideal — and, as such, there is room for improvement.

In the arts there was a broad movement beginning in the 19th century, realism (which led to naturalism), which sought to portray characters, scenes, and so forth, realistically. This was in contrast and reaction to romanticism,
which portrayed their subjects idealistically. Commentary about these
artistic movements is sometimes put in terms of the contrast between
the real and the ideal: on the one hand, the average, ordinary, and
natural, and on the other, the superlative, extraordinary, improbable,
and sometimes even supernatural. Obviously, when speaking in this
sense, "real" (or "realistic") does not have the same meaning as it
does when, for example, a philosopher uses the term to distinguish,
simply, what exists from what does not exist.

In the arts, and also in ordinary life, the notion of reality (or
realism) is also often contrasted with illusion. A painting that
precisely indicates the visually-appearing shape of a depicted object
is said to be realistic in that respect; one that distorts features, as
Pablo Picasso's
paintings are famous for doing, are said to be unrealistic, and thus
some observers will say that they are "not real." But there are also
tendencies in the visual arts toward so-called realism and more recently photorealism that invite a different sort of contrast with the real. Trompe-l'œil
(French, "fool the eye") paintings render their subjects so
"realistically" that the casual observer might temporarily be deceived
into thinking that he is seeing something, indeed, real — but in fact, it is merely an illusion, and an intentional one at that.

In psychiatry, reality, or rather the idea of being in touch with reality, is integral to the notion of schizophrenia, which has often been defined in part by reference to being "out of touch" with reality. The schizophrenic is said to have hallucinations and delusions
which concern people and events that are not "real." However, there is
controversy over what is considered "out of touch with reality,"
particularly due to the noticeable comparison of the process of
forcibly institutionalising individuals for expressing their beliefs in
society to reality enforcement. The practice's possible covert use as a political tool can perhaps be illustrated by the 18th century psychiatric sentences in the U.S. of black slaves for 'crazily' attempting to escape. See also anti-psychiatry and one of its prominent figures, the psychiatrist Thomas Szasz.

In each of these cases, discussions of reality, or what counts as
"real," take on quite different casts; indeed, what we say about
reality often depends on what we say it is not.

Reality, Worldviews, and Theories of Reality

Further information: World view

A common colloquial usage would have "reality" mean "perceptions,
beliefs, and attitudes toward reality," as in "My reality is not your
reality." This is often used just as a colloquialism indicating that
the parties to a conversation agree, or should agree, not to quibble
over deeply different conceptions of what is real. For example, in a
religious discussion between friends, one might say (attempting humor),
"You might disagree, but in my reality, everyone goes to heaven."

Reality can be defined in a way that links it to worldviews or parts
of them (conceptual frameworks): Reality is the totality of all things,
structures (actual and conceptual), events (past and present) and
phenomena, whether observable or no